The mean weight of six boys in a groups is 48 kg.

(c) 53 kg

Mean weight of six boys = 48 kg
Let the weight of the 6th boy be x kg.

We know :

Mean $=\frac{\text { Sum of all observations }}{\text { Total number of observations }}$

$=\frac{51+45+49+46+44+x}{6}$

$=\frac{235+x}{6}$

Given :

Mean $=48 \mathrm{~kg}$

$\Rightarrow \frac{235+x}{6}=48$

$\Rightarrow 235+x=288$

$\Rightarrow x=53$

Hence, the weight of the 6 th boy is $53 \mathrm{~kg}$.