The grammatical reason is that another is a determiner or pronoun. In this sentence you need an adjective and other is an adjective. Another is used to refer to one more of something that has already been referred to so I can see why it was a tricky question. I had to look that up to explain why. It was easy for a native speaker to tell which worked, but hard to find a reason. Here are some examples. His other house is in France. Refers to one of a pair of existing things. He built another house in France. Refers to getting one MORE of something. He has another house in France. Again refers to one MORE of something. Bring me another one. Again one more Bring me the other one. Bring me a different one.

A."Another" is the correct option. The difference between A and B is that "another" is used with a singular noun and "other" is used with uncountable and plural nouns. Here "garage" is a countable, singular noun. Example: "Mark went to play basketball with OTHER friends." I hope it helps!

Only B "other" is correct here. Tim's explanation is good. Mr. Scott has a garage in Silbury. His *another garage is in Pinhurst. [< grammatically wrong] [Mr. Scott has exactly two garages.] Mr. Scott has a garage in Silbury. His other garage is in Pinhurst. [Mr. Scott has two or more garages.] Mr. Scott has a garage in Silbury and (he has) another (garage) in Pinhurst.