Concept:A quadratic expression x2−4x+p is a perfect square when p equals the square of half the coefficient of x.Explanation:For a perfect square of the form (x−a)2=x2−2ax+a2.Compare x2−4x+p with x2−2ax+a2.Here, −2a=−4, so a=2.Then p=a2=(2)2=4.Alternatively, use the formula p=(2coefficient of x)2.Coefficient of x is −4.So p=(2−4)2=(−2)2=4.Thus, the expression becomes x2−4x+4=(x−2)2.Answer:p=4Correct option: C. 4