The correct option is D All of these
As it is elastic collision, kinetic energy is conserved.
i.e. KE1+KE2=KE′1+KE′2 ...(1)
Momentum will also be conserved, as there is no external force acting.
i.e. P1+P2=P′1+P′2
As, P2=0
∴ P1=P′1+P′2
Since, kinetic energy is given by, KE=P22m and P2=0⇒u2=0⇒KE2=0
From (1),
P212m=P′122m+P′222m
⇒ P21=(P′1)2+(P′2)2
Hence, option (D) is the correct answer.