I think it’s the third one, if not it’s the first.
1) alpha particle, the element Radium undergoes alpha decay, releasing an alpha particle.
Given that,
Horizontal velocity of the object, v = 20 m/s
Height of the cliff, h = 125 m
We need to find the time that it takes the object to fall to the ground from the cliff is most nearly. It can be calculated using second equation of motion. Let us consider that the initial speed of the object is 0. So,

Here, a = g and u = 0

So, the object will take 5 seconds to fall to the ground from the cliff.
40 km/h to in/s
60*60=3600 seconds in an hour
one km is about 0.621 miles
40 km is about 0.621*40=24.84 miles
5280 feet in a mile
12 inches on each foot
5280*12=63360 inches in a mile
63360*24.84=1573862.4 inches in 40km
40 km/h = 40*1573862.4/3600=17487.36 in/s