t = number of magazine subscriptions sold each week
"he sells at least 10 [magazine subscriptions]... each week" tells us that the smallest t can be is t = 10
Plug t = 10 into the function to get
f(t) = 48*t
f(10) = 48*10
f(10) = 480
The smallest amount of money Clarence earns is $480. This is the smallest value in the range.
The largest value in the range is 1200 because we plug t = 25 into f(t) to get
f(t) = 48*t
f(25) = 48*25
f(25) = 1200
Therefore the range is the set of multiples of 48 from 480 to 1200 (inclusive of both endpoints)
Answer: choice B
Side Note: Choice D is close to the answer but "the all whole numbers" needs to be "all multiples of 48"