Identify the reaction type

This is a typical dehydrohalogenation reaction of a haloalkane, where removal of $HX$ gives an alkene.

Under strong base / alcoholic conditions, elimination generally follows Saytzeff's rule: the more substituted and more stable alkene is formed as the major product.

Possible products from elimination

From the given options, the products are different isomeric dienes / cumulenes:

• A: $CH_2=CHCH_2CH=CHCH_3$
• B: $CH_2=CHCH=CHCH_2CH_3$
• C: $CH_3CH=C=CHCH_2CH_3$
• D: $CH_3CH=CH-CH=CHCH_3$

Compare stability of the possible products

Let us examine them:

• A: isolated diene
• B: conjugated diene, but one terminal double bond
• C: cumulated diene (allene type), less stable
• D: conjugated diene with both double bonds internal

Apply stability order

Stability order among such systems is generally:

$\text{conjugated diene} > \text{isolated diene} > \text{cumulated diene}$

Also, among conjugated dienes, the one with more substituted double bonds is more stable.

Therefore:

• D is more stable than B because D has two internal, more substituted double bonds.
• B is more stable than A because conjugation stabilizes the diene.
• C is least favored because cumulated dienes are less stable.

Major product

Hence the major elimination product is:

$CH_3CH=CH-CH=CHCH_3$

which is Option D.

Comparison with stored correct answer

Stored correct answer: D

Derived answer: D

They match.