Explanation:
A graph is a representation of data in a pictorial or visual form. It is used to display the relationship between two or more variables. There are various types of graphs, such as line diagrams, bar diagrams, pie diagrams, and pictograms. The correct option for this question is A, which is a line diagram.

Line Diagram:
A line diagram is also known as a line chart or a line graph. It is a type of graph that displays data as a series of points connected by straight lines. A line diagram is commonly used to display trends over time or to show the relationship between two variables.

Bar Diagram:
A bar diagram is also known as a bar chart or a column chart. It is a type of graph that displays data as rectangular bars. A bar diagram is commonly used to compare different categories or to show changes over time.

Pie Diagram:
A pie diagram is also known as a pie chart or a circle graph. It is a type of graph that displays data as a circle divided into segments. Each segment represents a different category, and the size of the segment is proportional to the percentage of data it represents. A pie diagram is commonly used to show the relative sizes of different categories.

Pictogram:
A pictogram is also known as a pictograph or a picture graph. It is a type of graph that uses pictures or symbols to represent data. A pictogram is commonly used to display data in a way that is easy to understand, especially for children.

Conclusion:
In conclusion, a graph is a pictorial or visual representation of data. There are different types of graphs, such as line diagrams, bar diagrams, pie diagrams, and pictograms. The correct option for this question is A, which is a line diagram.