Target question => How many of the guests were served a double scoop of ice cream?
Let the total number of guests = x

Statement 1 =>At the picnic, 60% of the guests were served a double scoop of ice cream
Guests served a double scoop of ice cream = 60/100 * x = 0.6x
The value of x is unknown. Statement 1 is NOT SUFFICIENT

Statement 2 => A total of 120 scoops of ice cream were served to all the guests at the picnic
All single scoop + all double scoop = 120
All double scoop = 120 - all single scoop
This does not answer the target question and statement 2 is NOT SUFFICIENT

Combining both statements together =>
From statement 1 => if 60% = double scoop then 100% - 60% = 40% of guests were given single scoop
60% of x + 40% of x = total number of scoops
0.6x + 0.4x = total number of scoops

From statement 2 => Total number of scoops = 120
2 * 0.6x + 0.4x = 120
1.20x + 0.4x = 120
1.6x/1.6 = 120/1.6
x = 75

The total guest that were served double scoop = 60% of x
= 60/100 * 75 = 45

Both statements combined together ARE SUFFICIENT
Answer = C