The correct option is A
The memo will be sent to the secretary by the manager.
The given sentence is in the active voice, as the subject 'The manager' performs an action 'will send' against the object 'the memo'.
If this sentence is changed to passive voice, then the object takes the place of the subject and vice versa and the word
'by' is introduced as now the subject receives the action by the object.
The verb form of the sentence is also changed from active form to
'Subject (The manager) + verb (will send) + direct object (the memo) + to + indirect object (the secretary)' changes to:
'New subject (The memo) + verb (will be sent) + to + indirect object (the secretary) + by + new object (the manager)'
passive form of 'will send' in the simple future tense is 'will be sent'.
This can be seen in option A, hence it's the right answer.
Option B: The verb form 'is sent' is in the present tense whereas the original sentence is in future tense 'will send'.
C: Similarly, in this option, 'has been sent' is in the present perfect tense, and not the required future tense.
D: The verb form 'had been sent' is in the past perfect tense whereas the sentence expresses the future.
Hence options B, C and D are incorrect.