The first unit took 10 hours and the fourth unit took 8.1 hours to complete. What is the learning curve?

A) 100%
B) 99%
C) 95%
D) 90%
E) 80%

To find the learning curve, we need to use the following formula:

Y = aX^(-b)

where Y is the time to complete a unit, X is the number of units completed, a is a constant representing the time it takes to complete the first unit, and b is the learning rate.

Let's use the given information to solve for b:

10 = a(1)^(-b)
8.1 = a(4)^(-b)

Dividing the second equation by the first, we get:

8.1/10 = (a(4)^(-b)) / (a(1)^(-b))
0.81 = 4^b
b = log(0.81) / log(4) ≈ 0.099

Therefore, the learning rate is approximately 0.099, which means that for every doubling of the number of units completed, the time to complete a unit decreases by about 9.9%. To find the learning curve percentage, we can use the following formula:

Learning curve % = (1-b) × 100
= (1-0.099) × 100
= 90.1%

Therefore, the best choice is D, 90%.