Why do terrorist groups often target infrastructure like transportation systems?

to increase economic activities
to prompt foreign countries to send aid
to generate fear and frustration within the civilian population
to make the citizens revolt against their government

To generate fear and frustration within the civilian population.

Explanation:

Although there is no official definition of terrorism, we can imply that terrorism are acts of violence that target more than the actual victims, but seek to generate fear and frustration in large spectrums of society to achieve political objectives. Therefor; terrorists target crowded infrastructure like transportation systems because it generates chaos and attention and because it is easier to blend in so they don't get caught before or after the attack.

reread your text and you will find the answer. stay focused ignore all your distractions.

explanation:

a stronger central government  

its what they are