Which of the following is not a perfectly inelastic collision?

$A$ heavy steel ball of mass greater than $1\, kg$ moving with a speed of $2\, m/s$ collides head-on with a stationary ping-pong ball of mass less than $0.1\, g$. The collision is elastic. After the collision,the ping-pong ball moves approximately with a speed of ......... $m/s$.

$A$ moving particle of mass $m$ makes a head-on elastic collision with another particle of mass $2m$ which is initially at rest. The percentage loss in energy of the colliding particle on collision is close to .................. $\%$

$A$ particle of mass $m$ moving with velocity $v$ makes a perfectly elastic collision with a simple pendulum bob of mass $m$. After the collision,the maximum height attained by the bob is:

$A$ light particle moving horizontally with a speed $v_1$ strikes a very heavy block moving in the same direction with a speed $v_2$. The collision is elastic. After the collision,the velocity of the particle is:

$A$ particle of mass $m$,moving with a velocity $v$,makes an elastic collision in one dimension with a stationary particle of mass $m$. During the collision,they remain in contact with each other for an extremely small time $T$. Their force of contact with time is shown in the figure. Then $F_0 =$