Final answer:
Inadequate weather conditions disqualify an alternate in IFR flight.
Step-by-step explanation:
The correct option that disqualifies an alternate in IFR (Instrument Flight Rules) flight is Inadequate weather conditions (Option 1). Alternates must have weather conditions that are suitable for landing in case the flight cannot land at the intended destination due to poor weather conditions. If the weather conditions at an alternate airport are also inadequate, the flight cannot proceed.
Although options 2, 3, and 4 (Unavailability of suitable approach procedures, Closed or unattended airport, and Insufficient fuel reserves) are important considerations in flight planning, they do not directly disqualify an alternate airport.