What is the change in oxidation number of Cr when CrO42- is acidified?

+4 to +6 (increase) +3 to +7 (increase) +6 to +6 (no change) +5 to +3 (decrease)

+6 to +6 (no change)

2CrO42- + 2H+ → Cr2O72- + H2O  In CrO42- and Cr2O72- , the oxidation state of Cr is +6.