Answer:
B) Deposit into Killian’s account, then figure out his commission, deduct that number to arrive at the final amount (due to the principal) and then write a check to the principal for the monies due to him, within 3 business days.
Explanation:
Killian can deposit the money in the principal's account, in an escrow account or even deposit it in the broker's trust fund, whatever suits his mind or has been previously agreed with the seller, but he cannot keep the money for himself. Killian can only collect his commission once the sale has been completed.
There is no such thing as commissions in advance, since there is always the possibility that the deal is not completed, e.g. the house might burn down, the banking system may completely collapse, the buyer or seller might die, etc. Even though the possibility of extreme events occurring may be very low, it still exists.