The rich businessman bought a luxury duplex apartment that must have cost a/an_________.

"An arm and a leg" = an extremely high price — standard idiom for expensive purchases like a luxury duplex.

Why other options fail:

• incredulous bombshell — nonsensical mix; "incredulous" means *disbelieving* (an attitude), and "bombshell" means shocking news, not money.
• entire legacy — not a standard idiom for cost.
• large bushelful — a "bushel" is a grain measure; doesn't refer to monetary cost.

Option 1: arm and a leg.