Secondary butyl group is

Option Answer: Option (b)
CH3–CH2–CH2–CH2–

A butyl group is a group derived from butane (C₄H₁₀) by removing one hydrogen (H).

There are different types of butyl groups depending on where you remove that hydrogen from.

What does "secondary" mean?

Secondary (2°) means the carbon that is attached to the rest of the molecule is connected to two other carbons.

So, you're looking for a structure where the carbon holding the rest of the chain is bonded to two other carbon atoms.

Option (a):
CH3–CH–CH2–CH3
|
(This is where it connects to the main molecule)

This is called a sec-butyl group (secondary butyl), because:

The carbon attached to the main chain (the middle one) is connected to:

one CH₃ (methyl group)

one CH₂CH₃ (ethyl group)

and the main molecule → making it bonded to 2 other carbon atoms = secondary.

So this is the correct answer!

Option (b): CH3–CH2–CH2–CH2–