The correct option is
D The boy was wearing socks.
We see four parts in this picture. Here, we have to look at the picture and find out what they were doing when the phone rang. The phone rings in all the four parts of the picture. In the first part labelled 15, a boy is taking a bath. In the second part labelled 16, two boys are playing chess. In the third part labelled 17, a boy is wearing socks. In the fourth part labelled 18, a boy is sleeping. Since the question is in the past tense, the answer should be in the same tense as well. We must look at the options and select the best answer.Option A - 'The boy has a bag' is incorrect as having a bag doesn't indicate any action. Also, it is not in past continuous tense. Hence, option A is incorrect.
Option C - 'The boy worn shoes' is grammatically incorrect as the sentence doesn't have an auxiliary verb. Hence, option C is incorrect.
Option D - 'The boy has been sitting' is correct according to the picture, but that's a secondary action. The primary action is: He is wearing socks. Also, the answer given here is not in past continuous tense. Hence, option D is incorrect.
Option B - Yes, the boy was wearing socks when the phone rang. The answer is in past continuous tense and the action can be seen in the third part of the picture, labelled 17. Hence, option B is correct.