In the model of dna, if the yellow peg represents cytosine, then the green peg must be a) adenine. b) guanine. c) thymine. d) uracil.

I suppose the yellow peg is facing the green peg, in which case the green peg must be guanine.

The bases are associated in pairs. In each pair there is always a purine associated with a pyrimidine. The association is by weak bonds: the hydrogen bonds, two bonds for an AT pair; three links for a GC pair. When we know the sequence of one strand we can deduce the sequence of the other strand. The two strands are oriented opposite to the 3 'and 5' ends (the strands are called antiparallel).

For Future users the answer is B.  Remember A=T  and C=G hope that helps if anyone needs it!

a, i am pretty sure.