Given that, a superadditive function $f(\cdot):$ $$f\left ( x_{1} +x_{2}\right )\geq f\left ( x_{1} \right ) + f\left ( x_{2} \right )$$
Since $x >1,$ a superadditive function can never be a decreasing function. So, optionc C and D can staright away be ruled out.
We can check options A and B by taking the value of $x_{1} = 2$ and $x_{2} = 3.$
- $f(x) = \sqrt{x}$
$f(2+3) \geq f(2) + f(3)$
$\implies f(5) \geq f(2) + f(3)$
$\implies \sqrt{5} \geq \sqrt{2} + \sqrt{3}$
$\implies 2.236 \geq 1.414 + 1.732$
$\implies 2.236 \geq 3.146 \;{\color{Red} {\textbf{(False)}}}$
So, correct answer should be $(A).$
Reference:https://en.wikipedia.org/wiki/Superadditivity