The correct answer is option 'B' - class size.

Explanation:
The upper and lower limits refer to the highest and lowest values in a data set or a particular class interval. The difference between these two values is known as the class size. Let's break down the explanation further:

1. What are upper and lower limits?
- In statistics, upper and lower limits are used to define the range of values within a data set or a specific class interval.
- The upper limit represents the highest value in the data set or the upper boundary of a particular class interval.
- The lower limit represents the lowest value in the data set or the lower boundary of a specific class interval.

2. What is the class size?
- The class size is the numerical difference between the upper and lower limits.
- It indicates the range or width of a particular class interval.
- It helps to determine the spread or variability of the data within that interval.

Example:
Let's consider a data set of student scores in a test, where the highest score is 95 and the lowest score is 50. The class size can be calculated as follows:

Class size = Upper limit - Lower limit
Class size = 95 - 50
Class size = 45

In this example, the class size is 45, which means that the data set spans a range of 45 points.

Relationship with other options:
a) Class interval: Class interval refers to the range of values that are grouped together in a histogram or frequency distribution.
c) Class mark: Class mark refers to the midpoint or average of a class interval.
d) Group: The term "group" is a general term and does not specifically refer to the difference between upper and lower limits.

Thus, the correct answer for the difference between the upper and lower limit is the class size (option B).