Which conditions are classified as IFR conditions?

Instrument Flight Rules (IFR) conditions are defined by specific visibility and ceiling criteria that generally require pilots to operate under instrument flight rules due to inadequate weather conditions for visual navigation.

In this context, conditions that are classified as IFR typically include situations where the ceiling is low and visibility is restricted. Specifically, a ceiling of less than 1000 feet combined with visibility of less than 3 statute miles clearly falls into the IFR category, as these conditions render it difficult for pilots to navigate visually and necessitate reliance on instruments.

The other options describe weather conditions that either meet or exceed the criteria for Visual Flight Rules (VFR), where pilots can navigate using visual references outside the cockpit. For instance, a ceiling of 2000 feet with visibility of 5 miles or even 4 miles exceeds the IFR limits, allowing pilots to operate under VFR. A ceiling of 1000 feet and visibility of 3 miles is on the edge but still allows for some visual navigation under VFR conditions. Therefore, the correct choice accurately represents conditions that require IFR operations, where pilots must depend on their instruments for navigation and control.