Circle the correct particle: 1. Locals have been warned to watch out for / about any suspicious activity in the area. 2. The park is rich of / in animal and plant life. 3. Parents play an important role in / at preventing their children from getting into trouble. 4. Please contact all members when you decide at / on the exact date and time of the meeting.

Solution:

This task requires selecting the correct preposition to complete idiomatic phrases or grammatical structures.

1. The phrase 'watch out for' is used to indicate caution about potential dangers or problems. Therefore, 'for' is the correct particle.
2. The phrase 'rich in' is used to indicate that something contains a large amount of something. Thus, 'in' is the correct particle.
3. The preposition 'in' is used with 'role' to indicate the area or context in which the role is played. 'An important role in preventing' is the standard phrasing.
4. The preposition 'on' is used when specifying a particular date or time for an event. 'Decide on the exact date' is the correct idiomatic expression.

Answer: 1. for, 2. in, 3. in, 4. on