Consider the following activities :
1. Identification of narcotics on passengers at airports or in aircraft
2. Monitoring of precipitation
3. Tracking the migration of animals

In how many of the above activities can the radars be used?

The correct option is Only two.

Let us analyze the application of radar (Radio Detection and Ranging) in each of the three given activities to understand why only two of them are standard and viable use cases:

1. Identification of narcotics on passengers at airports or in aircraft:
Radars are electromagnetic sensor systems used to detect, track, and locate objects at a distance by emitting radio waves or microwaves and measuring their reflections. While airports use millimeter-wave scanners (a type of active imaging system) for security screening to detect concealed weapons or metallic/non-metallic objects, radars are not capable of chemical identification to specifically identify "narcotics" (drugs) on passengers. Chemical detection of narcotics relies on other technologies such as trace detectors, ion mobility spectrometry (IMS), X-ray transmission, or trained canine units. Therefore, radar cannot be used for this activity.

2. Monitoring of precipitation:
Radar is extensively used for monitoring precipitation. Weather radar (specifically Doppler radar) transmits directional pulses of microwave radiation. When these pulses hit precipitation (such as rain, snow, or hail) in the atmosphere, a fraction of the energy is scattered back to the radar receiver. By analyzing the returned signals, meteorologists determine the location, intensity, type, and motion of precipitation. Thus, radar is used for this activity.

3. Tracking the migration of animals:
Radar aeroecology is a well-established field. Weather radars and specialized biological radars are widely used to monitor and track the migration of flying animals, including birds, bats, and insects, through the airspace. Radar allows researchers to observe migratory patterns, flight speeds, altitudes, and density of animal groups over large areas, especially during nighttime migrations. Thus, radar is used for this activity.

Consequently, radars can be used in exactly two of the listed activities: monitoring of precipitation and tracking the migration of animals.