Unsubstituted organic acids are weak (or) very weak acids. Which of the following compound is most acidic?

Electronegativity of ${\mathrm{C}}_{\mathrm{sp}}>{\mathrm{C}}_{{\mathrm{sp}}^2}>{\mathrm{C}}_{{\mathrm{sp}}^3}.$ 

Greater the stability of the anion after the loss of proton stronger is the acid.

More electronegative atom can hold negative charge most readily. 

$\begin{array}{l}\therefore \mathrm{stability}\mathrm{of}\mathrm{HC}\equiv \stackrel{⊝}{\mathrm{C}}>{\mathrm{CH}}_3-\mathrm{C}\equiv \stackrel{⊝}{\mathrm{C}}>{\mathrm{CH}}_2=\stackrel{⊝}{\mathrm{C}}\mathrm{H}>{\mathrm{CH}}_3-\stackrel{⊝}{\mathrm{C}}{\mathrm{H}}_2\\ \mathrm{stability}\mathrm{of}\mathrm{CH}\equiv \stackrel{⊝}{\mathrm{C}}>{\mathrm{CH}}_3-\mathrm{C}\equiv \stackrel{⊝}{\mathrm{C}} \mathrm{because}\end{array}$

+I effect of $-{\mathrm{CH}}_3$ group intensifies the negative charge and destabilizes the carbanion.