the correct answer is 25 N 
 
 
Given::  \(\mu_s\) = 0.2 
Solution:: 
m=0.5 kg 
g=10 m/s2 
We know that \(f_s= \mu N\) 
To keep the block adhere to the wall
here N = F ...(2)
fs = mg ....(3)
from equation (1), (2), and (3), we get 
\(= mg=\mu F\) 
\(\therefore F={mg\over\mu}\) 
\(F={0.5×10\over0.2}\)
F=25N