Which sonobuoys are not utilized in submarine detection?

The correct choice identifies the types of sonobuoys that are not used for submarine detection. Bathythermobuoys are specifically designed to measure temperature and sound speed in the water column, providing important environmental data which can influence the operational effectiveness of anti-submarine warfare but are not directly involved in detecting submarines themselves. Down-Link Communication buoys are primarily used to transmit data from other buoys or sensors, rather than for detection purposes.

In contrast, the other options contain buoy types that play critical roles in submarine detection. Directional buoys, for example, are used to pinpoint sounds in the water, making them essential for locating submarines. DICASS (Digital Improved Combat Anti-Submarine System) buoys actively search for submarines by emitting sonar pulses, and thus are directly tied to submarine detection efforts. Similarly, Communication and Navigation buoys serve different operational purposes related to aiding naval navigation but do not focus on the detection of submarines.