Explanation:

- "Hold onto" means to keep something or someone, both physically or abstractly. It's suitable for thoughts or ideas that you don’t want to forget or discard.

- Correct Answer: Hold onto

- "Hold to" means to stick to a principle, promise, or decision. It's less about physically retaining something and more about adherence.

- "Hold on" is generally used for waiting or pausing for a moment, often in conversation; it also can mean to persist.

- "Hold against" means to blame someone or resent them for something. It’s more about harboring negative feelings due to past actions.