Which is true?

A) An number is either even or odd. If it is even, it can be written as $n=2a$ and its square as $n^2=4a^2$ , which is clearly divisible by $4$ and gives remainder of )0). If the number is odd, it can be written as $2a+1$ and its square as $n^2=(2a+1)^2=4a^2+4a+1=4(a^2+a)+1$ , which gives remainder of $1$ if divided by $4$ . So the statement is true.

B) $4^1=4$ ends in $4$ . If the last digit $4$ is multiplied by $4$ , it gives last digit $6$ . If the last digit $6$ is multiplied by $4$ , it gives last digit $4$ . And so on. The last digits alternate between $4$ and $6$ , never reaching zero. So the statement is true.

C) If $n$ is not divisible by positive integers less than $\sqrt n$ , then it must be prime. It cannot be divisible by an integer larger than $\sqrt n$ , since dividing by this integer would have to produce integer smaller than $\sqrt n$ , and that would contradict the assumption.

D) All of the statements A), B), and C) are true, therefore D) is true.