1.I had to answer question A because it was.....
a)compulsory b)necessary c)a must d)an obligation
(In my humble opinion all of those seem to be suitable) They are all possible grammatically, but 'compulsory' is more usual.
2.He denied having killed anybody/nobody(?) Anybody.
3.A little knowledge is a dangerous thing.
(My friend claims that this sentence is correct, but I disagree: Little knowledge is... Am I right?) Your friend is right.
Someone else might help you with the rest, that is my contribution.