Step 1: Meaning of Section 34 IPC.
Section 34 deals with acts done by several persons in furtherance of common intention.
Step 2: Requirement.
To establish liability under Section 34, it is necessary to prove that there was a common intention among the accused, but it is not necessary to prove the specific overt act of each individual.
Step 3: Conclusion.
Thus, option (a) is correct.