Step 1: Work backwards:
Adipic acid has six carbons and two acid groups at the two ends. For a single alkene to give both, those ends must have been joined in a ring.
Step 2: Test the candidates:
An open-chain hexene splits into two separate fragments whose carbon counts add to six, so no single product has six carbons.
Cyclohexene splits only at the ring double bond, so the ring just opens into one six-carbon chain.
Step 3: Conclusion:
Only cyclohexene gives hexanedioic acid (adipic acid), which is the raw material for nylon-6,6.
Final Answer:
Cyclohexene is the alkene, option (D).
\[ \boxed{\text{Cyclohexene (D)}} \]