Analysis:
To answer this question, we need to carefully analyze the given information and determine who was sitting at the back of the dim class.

Given Information:
- The class is dimly lit.
- Someone is sitting at the back of the class.

Possible Answers:
a) a girl
b) an old man
c) a teacher
d) a young boy

Reasoning:
To arrive at the correct answer, we need to consider the given information and make logical deductions based on it.

1. The class is dimly lit.
- This implies that it is difficult to see clearly in the class due to low lighting.

2. Someone is sitting at the back of the class.
- This indicates that there is a person occupying the back row or area of the class.

Deduction:
Considering the given information and making logical deductions, we can conclude that the person sitting at the back of the dim class is most likely a young boy. Here's the reasoning behind this deduction:

- It is common for young boys to prefer sitting at the back of the class.
- Young boys are often more active and restless, and sitting at the back of the class allows them more freedom and space.
- Additionally, young boys may find it easier to engage in activities or distractions without being noticed by the teacher or other students while sitting at the back.

Therefore, based on these deductions, we can select option 'D' as the correct answer, indicating that a young boy was sitting at the back of the dim class.

Conclusion:
The person sitting at the back of the dim class is most likely a young boy.