Let consider the monthly income = x

Amount spend on house rent = $\frac{2x}{5}$

Amount spend on food = $\frac{3x}{10}$

Amount spend on conveyance =$\frac{x}{8}$

After expenditure, the left amount = 2800

According to the question,

$$\begin{gathered} = x-\left(\frac{2x}{5}+\frac{3x}{10}+\frac{x}{8}\right)=2800 \\ =x-\left(\frac{\left(16x+12x+5x\right)}{40}\right)=2800 \\ =x-\frac{33x}{40}=2800 \\ =\frac{(40x-33x)}{40}= 2800 \\ =\frac{7x}{40}=2800 \\ = 7x=2800\times 40 \\ =x=2800\times \frac{40}{7} \\ =x=400\times 40 \\ =x=16000 \end{gathered}$$

Expenditure on food$=3\times \frac{16000}{10}=4800$

Expenditure on conveyance =$\frac{x}{8}=1\times \frac{16000}{8}=2000$

Expenditure on food and conveyance together

= (4800+2000) = 6800

Hence, Option D is correct.