Hi Julia,
Since you posted only one question, may I ask you to share all the other questions for the benefit of all of us here?
For the question above, I believe the answer should be: B

• When I was in China, I took a nice photo of the Great Wall.

Option D is incorrect because photo is a count noun that should take the article a if singular (i.e. a photo), as in option B, or that should be in its plural form (i.e. photos).

• When I was in China, I took photos of the Great Wall.

Sub-clause “When I was in China” provides a specific time reference, so the main clause should in the simple past tense, not in the present perfect tense. Moreover, the relative pronoun when can also mean before or after and, in this case, it’s mostly used in the past perfect tense, for instance,

• When I went to China, I had already had a nice photo of the Great Wall. (when = before)
• When I went to China, I had already had some nice photos of the Great Wall. (when = before)
• When I had gone to China, I had some nice photos of the Great Wall. (when = after)