From an algebraic point of view, a prime implicant is an implicant
such that if any literal is removed from that term, it is no longer an implicant.
From that viewpoint, ABCD is a prime implicant because BCD,
ACD, ABD, and ABC are not implicants (that is, if we remove any
literal from that term, we get a term that is 1 for some input combinations
for which the function is to be 0). However, ACD is not a prime implicant
since when we remove A, leaving CD, we still have an implicant. (Surely,
the graphical approach of determining which implicants are prime implicants
is easier than the algebraic method of attempting to delete literals.)