The required "option d) 4845" is correct.
Explanation:
The number of students in the class = 20
Students to pick up books for the class = 4
Here n = 20 and r = 4
To find, the total number of combinations of 4 students can he choose = ?
∴


Using the formula,




= 4845
The total number of combinations of 4 students can he choose = 4845
Hence, the required "option d) 4845" is correct.