A bar graph is a type of chart that uses rectangular bars to represent and compare different quantities or categories. It is a visual tool that helps to present data in a clear and organized manner.

The correct answer to the question is option D, which states that a bar graph compares certain quantities. Let's explore this in detail:

Bar Graphs and Their Purpose:
- A bar graph is used to compare different quantities or categories by displaying them as bars of varying lengths or heights.
- The length or height of each bar represents the value or amount of the quantity being compared.

Comparing Certain Quantities:
- A bar graph is specifically designed to compare certain quantities. These quantities can be numerical data, such as numbers of items, amounts, or measurements.
- For example, a bar graph can be used to compare the number of students in different grade levels, the sales figures of different products, or the average temperatures in different cities.

Visual Representation:
- By using a bar graph, the data is visually represented through the length or height of the bars.
- The bars can be arranged vertically or horizontally, depending on the purpose and clarity of the graph.

Interpreting the Bar Graph:
- To interpret a bar graph, one needs to compare the lengths or heights of the bars.
- The longer or taller the bar, the larger the quantity it represents.
- The bars can be color-coded or labeled to represent different categories or groups within the data.

Other Types of Graphs:
- While bar graphs compare certain quantities, there are other types of graphs that serve different purposes.
- Line graphs, for example, are used to show the change or trends in data over time.
- Pie charts are used to represent proportions or percentages of a whole.

In conclusion, a bar graph is specifically designed to compare certain quantities. It visually represents data by using bars of different lengths or heights. The lengths or heights of the bars correspond to the values or amounts being compared.