The correct option is D 14.28 kg
Given: initial velocity, u=7 ms−1, final velocity, v=3u=21 ms−1, change in momentum, Δp=200 kgms−1.
Let the mass of the body be m. Then,
Inital momentum =mu
Final momentum =mv
Change in momentum,
Δp=Final momentum−Initial momentum⇒Δp=mv−mu=m(v−u)⇒m=Δp(v−u)
⇒m=20021−7=14.28 kg
Therefore, the mass of the body is 14.28 kg