What a sensation it would cause if it . . . . . . . now discovered that such a man had left behind a diary.

The correct answer is: C. was

The sentence is in the past tense, so the verb “to be” should also be in the past tense. The other options are incorrect because they are in the present tense.

Here is a brief explanation of each option:

• A. is is in the present tense. This is incorrect because the sentence is about something that happened in the past.
• B. be is also in the present tense. This is also incorrect for the same reason as option A.
• C. was is in the past tense. This is the correct answer because the sentence is about something that happened in the past.
• D. were is in the plural form of the past tense. This is incorrect because the subject of the sentence is “it,” which is singular.