CH₃ - CH- CH₂ CH₃

Optically Active Compounds
Optically active compounds are those that can rotate the plane of polarized light. They have chiral centers or asymmetric carbon atoms, which means they have four different groups attached to the carbon atom. When light passes through these compounds, it interacts with the chiral center and gets rotated in a specific direction.

Given Options
a) n-Propanol
b) 2-Chlorobutane
c) n-Butanol
d) 3-Hydroxypentane

Explanation
To determine whether a compound is optically active, we need to examine its structure and identify any chiral centers.

a) n-Propanol: This compound does not have a chiral center because the carbon atom attached to the hydroxyl group is also attached to two other carbon atoms. Therefore, it is not optically active.

b) 2-Chlorobutane: This compound has a chiral center because the carbon atom attached to the chlorine atom is also attached to three different groups (one hydrogen, one methyl group, and one ethyl group). Therefore, it is optically active.

c) n-Butanol: This compound also has a chiral center because the carbon atom attached to the hydroxyl group is also attached to three different groups (one hydrogen, one methyl group, and one ethyl group). Therefore, it is optically active.

d) 3-Hydroxypentane: This compound does not have a chiral center because the carbon atom attached to the hydroxyl group is also attached to four other carbon atoms. Therefore, it is not optically active.

Conclusion
Among the given options, only 2-Chlorobutane (option B) is optically active because it has a chiral center. The other compounds do not have chiral centers and are therefore not optically active.