Step 1: Break down the maxim.
Jus ad rem is a Latin phrase from jurisprudence about a particular kind of legal right.
Step 2: Explain its meaning.
Jus ad rem is a right to a thing that arises from an obligation or contract. It is an incomplete right to claim property.
Step 3: Compare with similar terms.
Jus in rem is a right against the whole world, and jus in personam is a right against a specific person. Jus ad rem sits between them.
Step 4: Match the option.
The best fit is a right to a thing.
Answer: Right to a thing (Option 3).