Question

5. A number n is said to be perfect if the sum of all
its divisors (excluding n itself) is equal to n. An
example of perfect number is
(a) 6
(b) 9
(c) 15
(d) 21​

Answer 1

Lets check each option one by one 

A. 9 divisor are 1 and 3. 

1+3=4=9 so not a perfect number.

B. 15 divisors are 1,3, and 5. 

1+3+5=9=15 so not  a perfect number

C. 21 divisors are 1, 3 and 7. 

1+3+7=11=21, so not a perfect number.

D. 6 divisors are 1,2 and 3. 

1+2+3=6 It is a perfect number 

So correct answer is option D