Why it's wrong? They may not be "the most expensive" at the market, but they are the best quality
A: We have the sweetest pears! Here, try some! They are seven for $600.
B: They're really sweet, but they're too expensive.
A: Well, they may not be _____ at the market, but they are the best quality.

(a) the cheapest (b)the cheaper (c) less expensive (d) the most expensive

the answer from the book is "(a) the cheapest" which I accept as a good answer, but I think "(d) the most expensive" is acceptable too.

In my opinion, (d) is like:
You think it's too expensive?! They are not too expensive. In fact, they are not the most expensive, but they are the best quality.

Please tell me why (d) is not acceptable.

If there is anything wrong in my opinion, please help to correct it. Thank you for your attention.

Question

Why it's wrong? They may not be "the most expensive" at the market, but they are the best quality
A: We have the sweetest pears! Here, try some! They are seven for $600.
B: They're really sweet, but they're too expensive.
A: Well, they may not be _____ at the market, but they are the best quality.

(a) the cheapest (b)the cheaper (c) less expensive (d) the most expensive

the answer from the book is "(a) the cheapest" which I accept as a good answer, but I think "(d) the most expensive" is acceptable too.

In my opinion, (d) is like:
 You think it's too expensive?! They are not too expensive. In fact, they are not the most expensive, but they are the best quality.

Please tell me why (d) is not acceptable.

If there is anything wrong in my opinion, please help to correct it. Thank you for your attention.

Answer

They are not the cheapest but they are the best quality is correct. The seller saying you can buy cheaper but these are the best quality.

D Is incorrect it is untrue, the seller would be saying the pears at  600 [currency unimportant here] for seven are the most expensive but then to claim they are not the most expensive at the beginning of the sentence.

A Is confirming the fact that the pears are a good quality but they may not be the cheapest.

Answer

The grammar is fine, but it’s false. Pears that cost $600 for seven clearly *are* the most expensive. Even if the currency is not the USD, the customer’s comment makes it clear that they are not the cheapest pears in the market. The correct answer is “They may not be the cheapest….” This is a common structure meaning the seller is admitting that they’re expensive, and giving a justification.

Answer

Dear @Layne deCooman
Thank you for your explanation
Your explanation is more understandable for me (as not a native speaker)

I agree with you, and understand where I was wrong,

But there is a point I don't understand --- that you mentioned [because with the "... market, BUT they are the best quality." would then argues with both points slightly]
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2. In my opinion, the meanings of "not the cheapest" and "not the most expensive" are very close. 
The ranges are large, both of them can mean cheap, neutral price, expensive, and any kinds of prices, but only not the highest or the lowest price.
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3. We say "the most expensive, and with the best quality."
 => They are not the most expensive, but they are the best quality. 
For me, it is reasonable and with no argument.

If there is anything wrong in my opinion, please help to correct it. Thank you for your attention.

Answer

Saying "Well, they may..." means that they're at least somewhat agreeing with you, but later she tries to say it's worth it for the quality.
Saying that "they may not be the cheapest" continues with that earlier point.
Saying "may not be the most expensive" argues with the first point, causing the entire message to feel like it's jumping back and forth, because with the "... market, BUT they are the best quality." would then argues with both points slightly.

(NOT A TEACHER JUST A NATIVE SPEAKER)