Answer:
*First draw out each compounds' structural formula (an image of the "shape and arrangement of the atoms in space"), then identify all functional groups in each compound.
*Add Tollens' reagent to all three, the compound which provides a silver mirror / precipitate, is compound B. This is because B is the only one to have an aldehyde group, which is oxidised by Tollens and gives a positive result. Ketones are NOT oxidised by Tollens and so give a negative result (no mirror).
*Next, add acidified potassium dichromate, formula Cr2O72-, to all three. A and B give a positive result for this test, a colour change from orange to green, as they both have either primary or secondary alcohols. C gives a negative result, no colour change, as it has a tertiary alcohol which won't be oxidised by the dichromate.
So:
Tollens = X, Cr2O7 = ✓ -> Compound A
Tollens = ✓ -> Compound B
Tollens = X, Cr2O7 = X -> Compound C