Answer
$H_2SO_4$ is stronger than $H_2SeO_4$ because, the strength of oxyacids depends on the electronegativity of the central atom. Since $S$ is more electronegative, $H_2SO_4$ is stronger.
Work Step by Step
This happens because, when the central atom of an oxyacid is more electronegative, it attracts the electrons from the hydrogen atoms, making the donation easier.
