c. 10 units

Reason:

A firm seeks to maximise its profit at the level where the MR = MC.In a perfectly competitive market, the firm will produce the level of output where the equality between MR and MC is attained where,

TR = Price * units of output

MR = TRn- TRn-1, where n depicts the number of units

MC= TCn - TCn-1

Profit = TR-TC

If we compute the values based on the above formulas, we see that, at 10 units of output, the MR = MC. At this level, the total revenue = $500, while the total cost = $320, which makes the total profit = $180. Hence we see that the firm will maximise its profit by producing 10 units of output.