The weight of 10 students (in kgs) are

(c) 42 kg

Arranging the numbers in ascending order, we have:
31, 35, 36, 38, 40, 44, 45, 52, 55, 60
Here, n is 10, which is an even number.
Thus, we have:

Median $=$ Mean of $\left(\frac{n}{2}\right)$ th observation \& $\left(\frac{n}{2}+1\right)$ th observation

Median weight $=$ Mean of the weights of $\left(\frac{10}{2}\right)$ th student \& $\left(\frac{10}{2}+1\right)$ th student

$=$ Mean of the weights of 5 th student & 6 th student

$=\frac{1}{2}(40+44)=42$

Hence, the median weight is $42 \mathrm{~kg}$.