Q$8.$ The process of daily realization minimizes the possibility of contract default. To ensure parties do not default the exchange requires a I posted in each account, and if the collateral in the account falls below a pre$-$specified level, the broker will make a

$($a$)$ settlement price; margin call I

$($b$)$ margin requirement; margin call

$($c$)$ margin call; margin requirement

$($d$)$ margin call; settlement price