Answer : Option C

Explanation :

Let the monthly income of A = a
monthly income of B = b
monthly income of C = a
$MF#%\begin{align}
&a + b = 2 \times 5050 ---------(Equation 1)\\\\
&b + c = 2 \times 6250 ---------(Equation 2)\\\\
&a + c = 2 \times 5200 ---------(Equation 3)\\\\
\end{align} $MF#%
(Equation 1) + (Equation 3) - (Equation 2)
$MF#%=> a + b + a + c - (b + c) = \left(2 \times 5050\right) + \left(2 \times 5200\right) - \left(2 \times 6250\right) $MF#%
=> 2a = 2(5050 + 5200 - 6250)
=> a = 4000
=> Monthly income of A = 4000