Explanation:

- Estoppel is a legal principle preventing someone from arguing something contrary to a previous stance if it would harm another who relied on the initial stance.

- Option 1: Civil action

- Estoppel is frequently applied in civil cases, like contract disputes or real estate matters.

- Option 2: Criminal action

- Estoppel is not typically associated with criminal proceedings, as these cases follow different procedural rules.

- Option 3: Both civil and criminal action

- Estoppel is predominantly related to civil cases, making this option not generally applicable.

- Option 4: Only (b) and not (a)

- This is incorrect, as estoppel is largely a civil concept.

Answer: Option 1 - Civil action