Explanation:

- Option 1: during the continuance of tenancy

- Estoppel can operate during the timeframe of an active lease.

- A tenant may be estopped from denying the landlord's title or rights during this period.

- Option 2: who remain in possession after the termination of tenancy by notice to quit

- Estoppel may also apply if a tenant remains in possession after the lease ends.

- Prevents the tenant from claiming adverse ownership against the landlord.

- Option 3: both (a) and (b)

- Covers both scenarios: active tenancy and holding over post-termination.

- Estoppel ensures tenants cannot contest landlord's rights in both cases.

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

- Suggests estoppel applies only while the lease is active.

- Ignores post-termination possession scenarios.

Option 3 is correct: both (a) and (b).