The correct option is D was submitted
Here, 'was submitted' in simple past tense is correct because it
indicates an action that has already been completed. Here, the 'memorandum 'was' (past tense) submitted.
'Were submitted' is incorrect because memorandum (subject) is singular, not plural. So, 'was' is correct.
'Was being submitted' in present continuous is incorrect because it implies an action going on at the time of speaking.
'Will be submitted' in future continuous tense is incorrect because it indicates what will be going on in the near future.