Determine the number of units the average worker can

Question

An efficiency study showed that the rate at which the average worker assembles products t hours after starting work can be modeled by the function

f ( t ) = - t² + 12 t + 15 ,  where 0 ≤ t ≤ 4

Determine the number of units the average worker can assemble during the fourth hour that s/he works during a shift.

a) 45 units

b) 8 units

c) 135 units

d) 2 units

e) 132 units

f) None of the above.