Which of the following turns K₂Cr₂O₇ paper green?

Sulfur dioxide is the gas that turns K₂Cr₂O₇ paper acidified with dilute H₂SO₄ to a green compound.

It is formed when sulfur is reacted with sulfuric acid.

$\mathrm{S}\left(\mathrm{s}\right)+2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)\to 3{\mathrm{SO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Due to the production of chromium (III)sulfate, Cr₂(SO₄)₃ the orange-colored dichromate solution will turn green.

${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)+3{\mathrm{SO}}_2\left(\mathrm{g}\right)\to {\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right)+{\mathrm{Cr}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

A redox reaction has occurred, Sulfur dioxide serves as the reducing agent and potassium dichromate as the oxidizing agent.