Step 1: Understanding Section 18.
Section 18 of the Indian Evidence Act, 1872 deals with admissions by parties, their agents, and others.
Step 2: Admissible admissions.
- Admissions by one of the plaintiffs are admissible against all plaintiffs.
- Admissions by one of the defendants are admissible against co-defendants.
- Admissions by agents of parties are also admissible.
Step 3: Non-admissible.
Statements by a third party (stranger to the case) are not admissible as evidence.
Step 4: Conclusion.
Thus, the correct answer is (D).