this is tough.

ok as non-missable i presume you mean can't be avoided, so its definitely not B, since its gotten by going to the Wutai thingy, which can be completely avoided. A... its at.... oh! after the red dragon. so its not A.

now C or D. IIRC, the only non-missable summon is Shiva, which is given by the little girl. so i say

D) No 2nd non-missable.