Among the following , a perfect number is

Perfect number : Sum of factors of a given number is equal to twice of that number.

Option A)

Factors of $6=1,2,3,6$

Sum of factors of  6$=1+2+3+6$

                                $=12$

    Twice the number$=2\left(6\right)=12$

Option B)

Factors of $28=1,2,4,7,14,28$

Sum of factor of 28 $=1+2+4+7+14+28$

                                $=56$

Twice the number $=2\left(28\right)$ $=56$

$\therefore$  6 and 28 are perfect numbers.

Option (c)

The factors of 100 are 1,2,4,5,10,20,25,50,100.

Sum of factors of 100$\text{=1+2+4+5+1}0+\text{2}0+\text{25+5}0+\text{1}00=271$

Twice the number $=2\left(100\right)$ $=200$

$\therefore$  271 is not twice of the 100.

Option (d)

 The factors of 625 are 1,5,25,125,625.

Sum of factors of 625$=1+5+25+125+625=781$

Twice the number $=2\left(625\right)$ $=1250$

781is not twice the625.

$\therefore$  Options c, d are not a perfect numbers.