Answer
(a) $H_2SeO_4$ is stronger.
(b) $H_3PO_4$ is stronger.
(b) $H_2Te$ is stronger.
Work Step by Step
(a) For oxoacids: More O atoms increases acidity.
(b) For oxoacids: The electronegativity of the central atom increases acidity. Since P is more electronegative than As.
(c) For Nonmetal Hydrides: Down a group, acid strength increases.
Since Te is below S in the periodic table, $H_2Te$ is more acidic.
