Put each set in order of increasing acid strength, and give the reason for each order…

(a) $\text{HF} < \text{HCl} < \text{HBr} < \text{HI}$. Down group 17 the atom grows. The H-X bond becomes longer and weaker, so the proton leaves more easily.

(b) $\text{CH}_4 < \text{NH}_3 < \text{H}_2\text{O} < \text{HF}$. Across period 2 the electronegativity rises from C to F. The bond to hydrogen becomes more polar, so $\text{H}^+$ is lost more easily.