Answer
(a) $H_2Se$
(b) $HI$
(c) $HNO_3$
(d) $H_2SO_3$
Work Step by Step
(a) $S$ and $Se$ are on the same group of the periodic table, therefore, the important factor is the bond strength. Since $Se$ is a bigger atom, the bond will be weaker, and its acid will be stronger.
(b) $I$ and $Te$ are on the same row, therefore, the polarity is the important factor. And since $I$ is more electronegative than $Te$, the polarity is greater, and the bond $H - I$ is weaker.
(c) The factor here will be the Oxydation number of the central atom: "N": For $HNO_2$, this number is +3, and for $HNO_3$, this number is +5. Therefore, $HNO_3$ is a stronger acid.
(d) The factor here will be the electronegativity of the central atom:
Since $S$ is more electronegative than $Se$, $H_2SO_3$ is a stronger acid.
