If Daryn earns $8 per hour, the value of the marginal product of the second hour of labor is $16.
The marginal product refers to the change in output value resulting from an additional unit of input.
The marginal product is given by the following formula:

MP = marginal product
= change in the firm's use of the input
= change in the quantity of output produced (ignoring external cost and benefits).
The amount earned by Daryn in the first hour = $64 (8 x $8)
The amount earned by Daryn in the second hour = $48 (6 x $8)
The marginal product = (64 - 48)/(2 - 1) = 16
= $16
Thus, we can conclude that the marginal product of the second hour of labor by Daryn is $16 not $48.