Introduction:
Optical isomers, also known as enantiomers, are non-superimposable mirror images of each other. They have the same chemical and physical properties, except for the way they rotate plane-polarized light. In this question, we are asked to determine the number of optically active isomers formed by C5H11Br.

Explanation:
To determine the number of optically active isomers of C5H11Br, we need to first identify the chiral centers in the molecule. A chiral center is a carbon atom that is attached to four different groups. In C5H11Br, there are two chiral centers, as shown below:

CH3-CH(Br)-CH2-CH2-CH3
|
CH3

To calculate the number of optically active isomers, we can use the formula 2^n, where n is the number of chiral centers. In this case, n=2, so the number of optically active isomers is 2^2 = 4.

Conclusion:
In conclusion, there are four optically active isomers formed by C5H11Br, due to the presence of two chiral centers in the molecule. It is important to note that the number of chiral centers in a molecule determines the number of optically active isomers, and the formula 2^n can be used to calculate this number.

There is no asymmetric carbon so no optically active isomer is possible