What preventive measure can be taken against terrorism?

The most effective preventive measure among the choices given is to let people close to you know where you are going. This action enhances personal safety by creating a system of accountability and support. When friends or family members are aware of your whereabouts, they can check on your safety and respond more quickly in case of an emergency. This communication can serve as an essential part of situational awareness, as it allows others to take action if you do not arrive at your destination as expected.

In contrast, the other options do not provide significant benefits in terms of improving personal security against terrorism. Avoiding all social activities can lead to isolation and does not address potential risks directly. Traveling only in large groups may offer some safety in numbers, but it is not a guaranteed way to prevent terrorism and can sometimes attract attention or create vulnerabilities. Changing your appearance regularly may reduce the likelihood of being recognized, but it is a less practical measure and does not ensure safety or prevent terrorist acts. Therefore, keeping close contacts informed of your plans is the most pragmatic and effective preventive approach in this context.