someone correct me if im wrong, but from my understanding:

you are buying 400 shares on margin, where the initial margin requirement is 50% and the initial price per share is $60. this means that the total amount costs

400 shares x $60 per share = $24000, where half the shares you acquire through leverage, that is, you only pay out of pocket $12,000 and borrow $12,000 worth of shares.

the maintenance margin is 25%, that is, 1/4 of the total,

or $24,000 x 0.25 = $6,000

so long as the borrowed shares are worth at least $6,000, there is no margin call.

when the shares fall to $40 per share, the borrowed shares are worth

200 shares x $40 per share = $8000, and since this is greater than the minimum maintenance margin of $6,000, there is no margin call.

thus B - there is no margin call because the investor still meets the minimum maintenance margin required

the critical price for the shares, that is the minimum value for which there is no margin call is then

$6,000 / 200 shares = $30 per share

so if the share price falls below $30, there will be a margin call and the buyer will be asked to top up his margin balance.

suppose the share price drop to $28 per share. the borrowed shares are now worth only 200 shares x $28 per share = $5,600 < $6,000

in this case, there will be a margin call and the investor is asked to deposit enough in their margin account to meet the initial margin requirement.

that is, they must deposit at least $12,000 - $5,600 = $6,400 by the end of the next trading day.

if the investor doesn’t put in the required variation margin, the broker closes the long position by selling the shares, and the investor loses

$12,000 - 400 x $28 = $800

if the price fell to $30 and the broker closed the account, the investor would lose

$12,000 - 400 x $30 = $0 as expected from calculations above, assuming i didn’t make any mistakes.