Final answer:
Reference lines are used to compare data points to a reference value in graphs, and can be horizontal, vertical, or diagonal. They are not exclusive to line graphs. The correct option is 1.
Step-by-step explanation:
In line graphs, reference lines are used to compare data points to a reference value. This helps to visually analyze how the data points deviate from the reference value.
Reference lines can be horizontal, vertical, or even diagonal, depending on the specific requirements of the graph.
While reference lines can be used in line graphs, they are not exclusive to this type of graph. They can also be used in other types of graphs, such as bar graphs or scatter plots, to provide a benchmark for comparison.
Hence, Option 1 is correct.