Ans: B.15
Let the number of MCQs be $x$ and number of essay questions be $y$.
Given MCQs are worth $3$ marks each and essay questions are worth $11$ marks each.
Also, total number of questions is $20$ worth $100$ marks in total.
$\Rightarrow x+y = 20$ and $3x+11y = 100.$
$\Rightarrow 3x+11(20-x) = 100$
$\Rightarrow 8x = 120$
$\Rightarrow x =\dfrac{120}{8} = 15$