Correct Answer - Option 2 : base line
Main line: The line joining main survey stations are known as Main survey lines.
Base line: The longest line that will divide the total area into two parts is known as Base line. This line is the main reference line for fixing the positions of various stations and also to fix the direction of other lines.
This should be carefully measured and laid as the accuracy of the entire triangulation critically depends on this measurement.
The lines which are run in the field to check the accuracy of the work are known as Check line. They are also known as Proof lines.