What will happen if a small amount of hydrochloric acid is added to a 0.1 m solution of hf?

pH will go down (become more acidic)

Explanation:

This is because HCl is strong acid that means it dissociates completely into H + and Cl - ions. There's already some H + ions in solution contributed by HF but addition of HCl supplies more H + which drives the pH of solution down making the solution more acidic.