SOLUTION :
Let the ages of Sachin and Amit be $7 x$ and $4 x$.
From the question,
$\begin{aligned}& 7 x+4 x=33 \\\Rightarrow & 11 x=33 \\\therefore \quad & x=\frac{33}{11}=3 \text { years }\end{aligned}$
Now, the Age of Sachin $=7 x$
$\begin{aligned}&=7 \times 3 \\&=21 \text { years } \\\& \text { the age of Amit } &=4 \times \\&=4 \times 3 \\&=12 \text { years }\end{aligned}$
$\text { Before } 6 \text { years their ages will be }$
$21-6=15$ years $\& 12-6=6$ years
$\therefore \quad$ Ratio $=\frac{15}{6}=\frac{5}{2}=5: 2 ;$ Ans.
$\therefore \quad$ Correct option is $(\mathrm{D})$