The question here is generally determined by how strongly polarised the bond of teh H which will be ionised is. I.e. generally when there is an o-h bond, then there is a greater chance of polarization. however in a c-h bond, the chance of h being ionised or even released is very low.
these kind of qsns can be answered by determining how strongly electron cloud is pulled away by the neighboring atom to the H atom to be released is.
As O is highly electronegative, O-H bond has a higher potential to release H in the form of H+. in case of COOH, the O-O-H, two negihboring atoms are highly electro-ve. Hence, this has even greater chance of being released as H+. However in CHO or CH3, C-H bond almost has electron cloud equidistant from both atoms. Hence there is little chance that H+ will be relased. Hence, relatively, COOH>OH>CH3 etc... hwoever, in case of C-H bond, if there is an electro-ve atom next to C(eg CHO grp) then O pulls on C and hence the electron cloud over C-H gets more distorted than in case of CH3. hence, CHO is more acidic than CH3.
this is the general strategy to answer this kind of qsns
