The correct order of heat of combustion for following alkadienes is:

Step 1: Understand the relationship between stability and heat of combustion

$\Delta H_{\text{combustion}} \propto \frac{1}{\text{stability}}$ More stable molecule → lower energy → lower heat of combustion.

Step 2: Compare stability of the three alkadienes

The three types of dienes:

• (a) Conjugated diene (alternate double bonds): More stable due to delocalization of $\pi$ electrons → most stable → lowest heat of combustion
• (b) Cumulated diene (allene) ($\ce{C=C=C}$): Least stable — strained geometry (two adjacent double bonds sharing central carbon) → least stable → highest heat of combustion
• (c) Isolated diene (double bonds separated by $\ce{CH2}$): Intermediate stability — no special resonance but no strain either → intermediate heat of combustion

Step 3: Order heat of combustion Let's look at the three conjugated dienes. First, diene has both double bonds in trans orientation. B has one in cis, one in trans, and the C has both double bonds in cis orientation. Trans orientation is more stable compared to cis. Answer: Option (a)

Key Points to Remember:

• Conjugated diene = most stable (lowest heat of combustion)
• Heat of combustion reflects relative stability
• More stable = lower internal energy = less heat released on combustion