b is correct. She was happy. This is something that happened in the past. She was happy when she finally saw her son. "She hadn't seen her son for three years" is something that happened before that. So you have something that happened in the past (was happy) and something that happened further back in the past (hadn't seen). To express that, we use past perfect (I had done, I had seen etc.) Hope that makes sense.

B is correct. The reason is tense agreement - 'was' and 'hadn't' are both in the past. Haven't is present, as in 'no, I haven't seen your son'.

I don't know if what I am going to say is right or not but I used to learn grammar by sense. if the lady didn't see her son till that moment so I will choose (B) if the lady didn't see her son but met him at the moment u mentioned and the effect of her son's arrival made her happy so I will choose present perfect tense (A) "she hasn't seen her son for 3 years" what I mean that we use the past perfect tense if it's impossible to change the action... she will never meet him again. We use the present perfect when we are talking about an event that started in the past and the event still has some influence in the present. so both tenses start in the past but past perfect tense also ended in the past.

Hi Sibru, haven't and hadn't are shortened versions of two words, 'have not' (haven't ) and 'had not' (hadn't). So hadn't would be the best option. You could also use hasn't (has not). I hope this helped.