The correct answer is option 4, which is 20 m/s. When a tennis ball is thrown vertically upwards, it will reach a maximum height and then start falling back down due to the force of gravity. The initial upward velocity determines the maximum height the ball can reach.

In this case, the ball reached a maximum height of 20 m. This means that the initial upward velocity was strong enough to propel the ball to that height before gravity pulled it back down.

Option 1, 8 m/s, would not be enough to reach a height of 20 m. The ball would not have enough initial velocity to overcome gravity.

Option 2, 12 m/s, would also not be sufficient to reach a height of 20 m. The initial velocity needs to be higher.

Option 3, 16 m/s, is closer to the correct answer, but still not enough. The ball would need a slightly higher initial velocity to reach 20 m.

Therefore, the correct answer is option 4, 20 m/s. This initial upward velocity would allow the ball to reach a maximum height of 20 m before falling back down.