From the section in volume six about options on futures:
“If the holder exercises the call and establishes a futures position, he earns interest on the futures margin account.”
I’ve never had a margin account so trying to figure this one out.
Futures have margin accounts that pay interest. If the margin is, say, 50% and you have a $100,000 futures position, then you have to post $50,000 margin. (Most people post this in T-bills, which earn interest.)