Step 1: Break the phrase.
"Onus probandi" is a Latin maxim. "Onus" means burden and "probandi" means of proving. Together it means burden of proof.
Step 2: Understand the rule.
The person who makes a claim must prove it with evidence. The court does not accept a claim just because someone says it.
Step 3: See how it works.
If a person claims to own land, that person must show sale deeds or records. In a criminal case, the prosecution must prove the guilt.
Step 4: Match Option A.
Option A says it is the burden of proof placed on the party making an affirmative claim. This is exactly what onus probandi means.
Step 5: Rule out the others.
Option B is the fact in issue. Option C is the standard of proof in criminal cases. Option D is the actual evidence. None of these define onus probandi.
Step 6: Final answer.
Onus probandi means burden of proof.
\[ \boxed{\text{Burden of proof which places the responsibility on the party making an affirmative claim to substantiate it with evidence.}} \]