Explanation:

- Estoppel by Silence: This occurs when a person’s silence leads another to believe in a certain state of affairs, influencing them to act to their detriment.

- Estoppel by Negligence: This arises when a party's negligence causes another party to change their position to their disadvantage.

- Estoppel by Election: This involves choosing between two inconsistent positions, where the choice made precludes the possibility of asserting the other.

- All the Above: The concept of estoppel can manifest in multiple ways, including by silence, negligence, or election.

Answer: Option 4 - all the above.