It was he not me, who decided to postpone the program.
(a) he not I
(b) him not me
(c) he not mine
(d) him not I
It should be option a) he not I
Reason : the correct word to use would be I rather than me as I is the subject of the sentence.
Also good approach is to divide the sentence and see it individually whether they make sense or not.
Eg It was I, who decided to postpone the program.
It was he, who decided to postpone the program.