The correct option is
B They were playing a game.
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 past tense, the answer should be in the same tense too. We must look at the options and select the best answer.Option A - 'They are seeing a photo' is incorrect, as no one in the picture is spotted seeing a photo. Hence, option A is incorrect.
Option C - 'They had been playing a game' is incorrect, as we generally use the past perfect continuous tense for a long-term action. We always use simple or continuous tense to describe a picture, a book or a movie, and as the question is in past continuous tense, we must answer in the same tense. Also, we do not know the duration of their game. Hence, option C is incorrect.
Option D - No one in the picture eats a pizza. Hence, option D is incorrect.
Option B - 'They were playing a game' is correct, as we see two boys are playing chess in the second part of the picture, labelled 16. Also, the answer is in past continuous tense. Hence, option B is correct.