200 g of water is heated and its temperature goes from 280 k to 300 k. what is the mass?
a. 200 g
b. 200 kg
c. 200 n
d. 20 g

You just told us that it's 200g of water. So it doesn't matter what happened to it. If you were telling us the truth right at the beginning of the question, then the mass of the water is 200g.

Deduce law of conservation of momentum from Newton's third law of motion. Consider two bodies A and B of masses mA and mB moving in the same direction along a straight line. ... Thus, total momentum of the two bodies is conserved provided no external force acts on them. This proves the law of conservation of momentum.

Hope this helps you