Correct Option: A
Let $X_{ij}$ be an indicator random variable, such that
$X_{ij}=\left\{ \begin{array}{rcl} 1 & \text{if collide} \\ 0 & \text{otherwise} \end{array}\right. $
$\therefore$ Probability that $i$ and $j$ both hash to the same slot.
$P_r(X_{i,j})=\frac{1}{n}$
$\implies E[X_{i,j}]=\frac{1}{n}$
Now,
$\text{E[No. of Colliding pairs]}= E\left[\displaystyle\sum_{i=1}^n \sum_{j=i+1}^n X_{i,j}\right]$
$\qquad \qquad \qquad = \displaystyle\sum_{i=1}^n \sum_{j=i+1}^n E[X_{i,j}]$
$\qquad \qquad \qquad =\displaystyle\sum_{i=1}^n \sum_{j=i+1}^n\frac{1}{n}$
$\qquad \qquad \qquad =\dfrac{n(n-1)}{2n}$
$\qquad \qquad \qquad =\dfrac{(n-1)}{2}$