Hydrogen fluoride gas (HF) and sodium hydroxide (NaOH) react in a test tube. They form water and sodium fluoride (NaF). Which type of reaction occurred in the test tube?

This reaction is a double replacement reaction. Hydrogen in the hydrogen fluoride gets replaced by sodium of the sodium hydroxide. HF is a weak acid and NaOH is a strong base. So, double replacement produces sodium fluoride and water molecules.