Question

They discussed ... the border issue at the Summit
(a) about (b) of
(c) against (d) none

Answer 1

They discussed about the border issue in the summit.

The use of about here makes the sentence complete, no sentence can ever be complete without the inclusion of a preposition.

Although about most of the times is interchanged with on, here, primarily this was the apt preposition available to choose from.

Answer 2

" a "

Sentence :

They discussed about the border issue at the summit.