No zoologist . . . . . . . . able to keep a gorilla under close observation in the dark jungles in which he . . . . . . . ..

Answer: Option D

In the given sentence "No zoologist is able to keep a gorilla under close observation in the dark jungles in which he had been living," the most appropriate words to fill in the blanks are "is" and "had been living."

"No zoologist is able to keep a gorilla under close observation" - The verb "is" is in the present tense, and it indicates a general truth or fact about the current situation.

"in the dark jungles in which he had been living" - The phrase "had been living" is in the past perfect continuous tense and indicates a past action that was ongoing before another past action. In this context, it refers to the gorilla living in the dark jungles before the observation took place.

The other options do not use the correct verb forms for the given sentence. Option A, "was/lived," uses the simple past tense for both verbs, which does not convey the correct sequence of events. Option B, "will/will live," uses the future tense for both verbs, which is not suitable for describing past events. Option C, "had been/has lived," uses the past perfect continuous tense and the present perfect tense, but they do not match the context of the sentence. Option E, "has been/lives," uses the present perfect continuous tense and the simple present tense, which also do not fit the context.

Therefore, the correct answer is "D. is/had been living.