Explanation:

- Overture: An introduction to something more substantial. It often refers to a preliminary part of an event or a piece of music.

- Option 1: Slander: Making false spoken statements damaging to a person’s reputation. This does not relate to introductions.

- Option 2: Conclusion: The end or finish of an event, process, or text. This is quite the opposite of an overture.

- Option 3: Preamble: An introduction or preliminary statement, especially in a document. This is similar to an overture.

- Option 4: Plunk: Generally refers to setting down something heavily or carelessly, unrelated to introductions.

Preamble is the correct synonym for overture. Both are introductory elements.

Option 3: Preamble is indeed the correct answer.