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 a perfect number is

Lets check each option one by one 

A. $9$ divisor are $1$ and $3$. 

$1+3=4\ne 9$ so not a perfect number.

B. $15$ divisors are $1,3,$ and $5$. 

$1+3+5=9\ne 15$ so not  a perfect number

C. $21$ divisors are 1, 3 and 7. 

$1+3+7=11\ne 21$, 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