If the number of pies will be enough for the guest to have at least one slice.
Guest=3; x>0 and 1 pie = 8 slices
Statement 1=> 5<2x<12
Dividing through by 2 gives;
$$2.5<x<6$$
Thus, x cab be either 3,4 or 5.
1 pie = 8 slices if x=3, then 8 * 3=24 => not enough
If x=4, then 8 * 4 = 32 => enough
If x=5, then 8 * 5 = 40 => enough
The information is not enough to arrive at a definite conclusion. Hence, statement 1 is INSUFFICIENT

Statement 2=> x is a multiple of 3
Since x>0, then x can be 3,6,9,12,15 etc.
If x=3, then 8 * 3 = 24 => not enough
If x=4, then 8 * 12 = 96 => enough
The information given is not enough to arrive at a definite conclusion. Hence, statement 2 is INSUFFICIENT

Combining both statement together;
From statement 1, x=3,4,5
From statement 2, x is a multiple of 3. The only multiple of 3 between 3,4 and 5 is 3. So, x=3
However, if one guest must have at least one slice of pie, then the pie will only be enough for 24 guests. Therefore, both statement combined together are SUFFICIENT.

Answer is option C