The correct order of stability of the carbocations A-C is

Concept:

Stability of Carbocation:

• The stability of a carbocation depends on several factors like hyperconjugation, resonance, and inductive effect. 
• Out of these factors, the resonance effect is the most important factor for the stability of a carbocation.
• The observed order of stability for carbocations is as follows:

tertiary>secondary>primary>methyl.

Explanation:

• In carbocation A, the positive charge resides on an sp hybridized C atom. An sp hybridized orbital destabilizes the positive charge. This is why carbocation A is the least stable among the three carbocations.
• Whereas for carbocation B and C, the positive charge resides on an sp2 hybridized C atom. The carbocation B is stabilized due to both hyperconjugation and inductive effect of the Ethyl (-CH₂CH₃) group.

• Carbocation C is the most stable carbocation among all the three carbocations as it is a resonance-stabilized carbocation.
• All the resonating structures of carbocation C are equivalent. As equivalent resonating structures are more important than non-equivalent resonating structures, carbocation C is the most stable carbocation.

Conclusion:

Therefore, The correct order of stability of the carbocations is C > B > A.