Answer:
chlorous acid HClO₂
Explanation:
The Ka is the acidity constant and it tells you about the acidity strength of a compound. If the value of Ka is high the compound is a strong acid. If the value of Ka is low the compound is a weak acid.
The problem gives us the following compounds with the Ka values:
HF, 3.5 × 10⁻⁴
HCOOH, 1.8 × 10⁻⁴
HClO₂, 1.1 × 10⁻²
HCN, 4.9 × 10⁻¹⁰
HNO₂, 4.6 × 10⁻4
The chlorous acid HClO₂ have the highest Ka, 1.1 × 10⁻², so this one is the strongest acid.