Correct answer: D — $84.00

Short sale value at initiation:

• $500 \times 70 = 35{,}000$

Initial margin account balance:

• $1.50 \times 35{,}000 = 52{,}500$

Margin call occurs when the account falls to the maintenance requirement, which is:

• $1.25 \times$ current market value of the shares short

Let the stock price be $P$. Then the current value of the short position is:

• $500P$

Set the account balance equal to the maintenance requirement:

• $52{,}500 = 1.25 \times 500P$
• $52{,}500 = 625P$
• $P = 84$

So further margin is required when the share price reaches $84.00.