Which of the following topics would lend itself only to a formal style writing Folk songs you have sung b psychological effects nuclear weapons c how lost 10 pounds d why living in cou?

The topic that would lend itself *only* to a formal style of writing is (b) psychological effects of nuclear weapons.

Here's why:

* Folk songs you have sung: This topic could be written in a casual, personal style or a more formal, analytical style, depending on the purpose.

* How I lost 10 pounds: This is typically a personal experience and could be written in a casual, informal style.

* Why I'm living in a country: This topic could be written in a variety of styles, from a personal narrative to a more analytical exploration of the reasons for the move.

The psychological effects of nuclear weapons is a serious and complex topic that typically requires a formal, objective, and scholarly approach. It involves discussing sensitive issues, scientific evidence, and potential societal impacts, all of which necessitate a formal style of writing.