Let the price of stereo X = x
Let regular price of stereo Y = y
Is the christmas sale price for stereo X lower than that of stereo Y ?

Statement 1
Sale price of stereo X is 15% less than regular price and stereo Y is 20%less than regular price
$$x-15\%\ of\ \ x$$
$$y-20\%\ of\ \ y$$
value pf x and y are unknown, hence statement is INSUFFICIENT.

Statement 2
Sale price of stereo X is 150 dollar less than its regular price
Sale price of stereo Y is 200 dollar less than its regular price
x-150
y-150
value of x and y are not known , hence statement 2 is INSUFFICIENT.

Combining both statements together.
$$\frac{Discounted\ amount}{Original\ amount}=\%\ discount$$
$$\frac{x-150}{x}=\frac{15}{100}=176$$ $$\frac{y-200}{y}=\frac{20}{100}=250$$
price of stereo X is lesser than stereo Y
both statements together are SUFFICIENT.

$$answer\ is\ Option\ C$$