Latin 'agendum' is a gerund, a verb in the form of a noun which conveys the meaning of obligation. Thus a 'memorandum' is 'a thing which must be remembered' , from the verb 'memorare' , 'to remember', and 'a thing which must be done' is an 'agendum', from 'agere', 'to act, to do'. The plurals end in 'a' for 'um' so 'memoranda' and 'agenda'.
It's curiously pedantic to give 'agendum' as the correct answer. It's a quiz question on English, not Latin, and nobody ever talks of the agendum, or the first item in an agenda as 'the first agendum'. It's a wonder that the quizmaster didn't insist upon the Latin pronunciation, with a hard 'G'.