is the question, I'm wagering my $ on c)
Because:
CH3CH3 doesn't want to lose any H+ because carbocations are highly unfavorable, which would mean its most likely the weakest acid
Molecule III is the strongest acid because its postively charged Oxygen is highly unfavorable, and if it loses an H it would become highly stable. Thus making it the strongest (though not necessarily strong in a technical sense) acid.
I'm also guessing that Molecule II is the second strongest because the OH group can lose an H and become resonance stabilized, making its conjugate base very weak. Even though the O has a negative charge, its also highly electronegative, so i guess its OK. Thus implying that the acid form is stronger.
Water is just water, i'm not really sure how to explain it.. but it was the only one left. It also has a pretty high pKa (~15.74)...