The constellation diagrams for a 4-ary modulation using PSK (qpsk),MSK ,QAM and OQPSK look alike. How to differentiate them?
In general, any modulation that makes sense to plot on a constellation diagram could be described as a kind of QAM. However, we generally use the term QAM only for modulations that use both degrees of freedom on the plot (amplitude and phase).
If the modulation varies the phase but not the amplitude — all the constellation points lie on a circle about the origin — then we call it PSK (phase-shift keying).
If the modulation varies the amplitude, but not the phase — all the constellation points lie on a ray from the origin — then we call it ASK (amplitude-shift keying).
So you can tell QAM from PSK because QAM displays multiple amplitudes as well as multiple phases, whereas PSK has exactly one amplitude. MSK and OQPSK are subtypes of PSK and you are right to think that you cannot tell them apart on a constellation diagram, but the question you show is not asking you to do so — only to distinguish between ASK, PSK, and QAM.