What is the velocity of the rock at 4.0s

The acceleration of gravity is 9.8 m/s².  That means that when a rock
is falling, its speed keeps increasing, and is always 9.8 m/s faster than
it was 1 second earlier.

If you just let it roll out of your hand, then it starts from zero speed, and
after 4 seconds, its speed grown to (4 sec) x (9.8 m/s²) = 39.2 m/s .
If there's no wind blowing it sideways, then its velocity is
39.2 m/s straight down.

Its AVERAGE speed during the 4 seconds is (1/2) (0 + 39.2 m/) = 19.6 m/s .

Falling for 4 seconds at an average speed of 19.6 m/s,
it falls    (4 sec) x (19.6 m/s)  =  78.4 m .
If there's no wind blowing it sideways, then its displacement is
78.4m straight down.

The correct answer is  "c".  it is "c" because melting is an endothermic change, and endothermic get's energy from surroundings.