The cluster's height above the valley floor can be modeled by the equation:

When the cluster seeds will reach the valley floor, the height of the cluster seeds above the valley floor will be 0. So substituting h(t) = 0 in the above equation we can find the time when the cluster seeds will reach the valley floor.

Since t represents the amount of time, it cannot have a negative value. So the only acceptable value of t is t=20
Thus it will take 20 seconds for the cluster of seeds to reach the valley floor. Thus the correct answer is option C