The carbon-carbon single bond length in propene ( CH_3-CH=CH_2 ) is...

In propene ($CH_3-CH=CH_2$), the alkyl group ($CH_3$) is attached to an $sp^2$ hybridized carbon atom of the double bond.

The $\sigma$ electrons of the C-H bonds in the methyl group undergo delocalization with the adjacent $\pi$ electron system of the double bond. This phenomenon is known as hyperconjugation or no-bond resonance.

Due to this hyperconjugative electron delocalization, the carbon-carbon single bond ($C_2-C_3$) acquires a partial double bond character. As a result, the bond length decreases from the standard C-C single bond length of $154 \text{ pm}$ in propane to approximately $150 \text{ pm}$ in propene.

Answer: Hyperconjugation