The correct option is
C Was given
The phrase 'last month' in the sentence indicates that the event has already happened, which means that the event is of the past.
The structure of the sentence indicates that the sentence is in the passive
voice.
Option A is in the present tense. This is not consistent with the phrase 'last month', hence, it is incorrect.
Option B is in the past tense. This indicates that the event of giving diary is an event of the past. Hence, being consistent with the phrase, this is the correct answer.
Option C is grammatically incorrect. Though it could be correct since this also indicates that the event has happened, but because it says 'have been given', this is grammatically incorrect. With 'a diary', it should be 'has been given'.
Option D is in the future tense, which is not consistent with the phrase 'last month'. Hence, it is incorrect.