The correct order of these functions is $f_1(n)< f_4(n)< f_3(n)< f_2(n)$.
The variable $n$ never appears in the formula for $f_1(n)$, so despite the multiple exponentials, $f_1(n)$ is constant.
Hence, it is asymptotically smaller than $f_4(n)$, which does grow with $n$.
We may rewrite the formula for $f_4(n)$ to be $f_4(n)=n \sqrt{n}=n^{1.5}$.
The value of $f_3(n)={}^{n}C_{2}$ is given by the formula $n(n-1) / 2$, which is $\Theta\left(n^2\right)$.
Hence, $f_4(n)=n^{1.5}=O\left(n^2\right)=O\left(f_3(n)\right)$.
Finally, $f_2(n)$ is exponential, while $f_3(n)$ is quadratic, meaning that $f_3(n)$ is $O\left(f_2(n)\right)$.
Answer : A