Which of the following is necessary for conducting IFR operations at uncontrolled airports?

For conducting IFR operations at uncontrolled airports, having an approved source of weather is essential. This requirement is primarily focused on ensuring that pilots have access to the latest and most accurate weather information necessary for safe flight operations under instrument flight rules. Weather can significantly impact visibility, cloud cover, and other conditions that are critical for decision-making during IFR operations.

Without reliable weather data, pilots may not be able to accurately assess the conditions they will encounter during departure, approach, and landing phases, potentially compromising safety. Access to an approved weather source allows pilots to comply with the regulations and make informed decisions when operating in such environments where air traffic control services are not available.

A radar system, an operating control tower, and a flight following service, while beneficial in certain contexts, are not mandatory requirements for conducting IFR operations at uncontrolled airports. Pilots can operate safely under IFR with just the necessary weather information, confirming that option B is the correct choice.