Step 1: Understand the meaning of the idiom.
The idiom "jump on the bandwagon" means to join or adopt something that is popular or trending, often because others are doing the same.
Step 2: Analyze the options.
Option (1): "To join a popular trend or activity" correctly reflects the meaning of the idiom, as it refers to participating in something popular.
Option (2): "To criticize something unfairly" is unrelated to the idiom.
Option (3): "To repair a vehicle" is also unrelated.
Option (4): "To start a business" does not match the meaning of the idiom.
Step 3: Conclusion.
The correct meaning of "jump on the bandwagon" is:
\[
\boxed{\text{To join a popular trend or activity}}
\]