Final answer:
The marginal change in the predicted value î for each unit change in x in the least-squares line î = 5 + 6x is 6.
Step-by-step explanation:
In the least-squares line î = 5 + 6x, the coefficient of x represents the slope of the line, which is also interpreted as the marginal change in the predicted value î for each unit change in x. Therefore, the marginal change in î for each unit change in x is 6. When x increases by 1 unit, î increases by 6 units. So the correct answer is b) 6.