Among the following numbers, a perfect square is

A perfect square number is always expressible as the product of pairs of equal factors.

Option ( A ) : $141=3\times 47$ .

$141$  can’t be expressed as the product of pairs of equal factors. So it is not a perfect square number.

Option  ( B ) : $196=2\times 7\times 2\times 7$ .

$196$ can  be expressed as the product of pairs of equal factors. Therefore$196$  is a perfect square number.

Option  ( C ): $124=2\times 2\times 31$ .

$124$  can’t be expressed as the product of pairs of equal factors. So it is not a perfect square number.

Option ( D ) : $122=2\times 61$ .

$122$  can’t be expressed as the product of pairs of equal factors. So it is not a perfect square number.