If a plant produce green coloured fruit in summer squash, its genotype would be:
WwYy
wwyy
WWYY
WwYY
As discussed in Dominant Epistasis -
In summer squash, the fruit colour is governed by two genes. The dominant gene W for white colour suppresses the expression of the gene Y which controls yellow colour. So yellow colour appears only in the absence of W. Thus W is epistatic to Y. In the absence of both W and Y, green colour develops. Hence, the correct option is (b).