A perfect number among the following is

A number for which sum of  all the factors is equal to twice of  the given  number is called a perfect number.

factors of 6 are $1,2,3,6$

Sum of factors of $6=1+2+3+6=12=2\times 6$

Factors of 28 are $1,2,4,7,14$  and 28

Sum of the factors of $28=1+2+4+7+14+28=56=2\times 28$