First D′ forces D=1 for the complemented expression to be one. Under D=1, the remaining inner terms are AB′+A′C+AC′+A′C′ = AB′+A′+AC′. Their complement requires A=1, B=1 and C=1. Together with D=1 this leaves only minterm ABCD, so the count is one.
More Boolean Algebra and Minimization questions
GATE 2015 CS (CS3) Q16– Define X\#Y=X+Y on Boolean variables. S1: (P#Q)#R=P#(Q#R). S2: Q#R=R#Q. Which hold?