Let shailesh's monthly income be Rs. x,

$\begin{array}{l}\text{Mehul's monthly income}=Rs\frac{15x}{8}\\ \\ \text{Prashant's monthly income}=Rs\frac{15x}{8}\\ \\ \therefore x+\frac{3x}{2}+\frac{15x}{8}=183750\\ \\ \Rightarrow \frac{8x+12x+15x}{8}=183750\\ \\ \Rightarrow \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}35}x\text{ = 183750}\times \text{8}\\ \\ \Rightarrow x=\frac{\text{183750}\times \text{8}}{35}\text{ = Rs 42000}\\ \\ \therefore \text{ Prashant's monthly income}\\ \\ \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=Rs}(\frac{15}{8}\times 42000)=Rs78750\end{array}$

Hence, option B is correct.