Answer:
Harvey's Hardware
Marginal cost of producing the third lawnmowers:
d. $100
Explanation:
Harvey's marginal cost for producing the third unit of lawnmowers is the additional cost that resulted when the total cost increased from $200 to  $300.  However, it can be deciphered from the case that the marginal cost for Harvey, which it is supposed to be  a variable cost, is traceable to the direct labor costs.  This implies that the fixed cost element for Harvey in the production of the lawnmowers has been relatively fixed at $100.  It does not vary with the volume of production, while the direct labor costs vary with the volume of lawnmowers produced by Harvey.