suppose that you measure a 480 count per minute (cpm) on your radiation detector placed 6 m from that radiation source , if you moved?

No. of cpm is inversely proportional to the distance between the detector and the radiation source.
No. of cpm ∝ (1/distance)²

When the detector is moved to a distance of 4 m, No. of cpm
= (480 cpm) × (6/4)²
= 1080 cpm

The answer: C) 1080 cpm

Inverse square law problem
I1/I2=D2^2/D1^2
Or simplified as the previous answer.
It still comes up to 1080. The meter was moved 1/3 the distance to the source. Which will increase the rate by 1.25 times. This is a given, just as surely as cutting the distance by 1/2 will give you a dose rate four times greater.