Answer : 4. "Gridline"
Explanation :

1. The horizontal lines displayed in the plot area of a chart in MS Excel 2010 are called gridlines.

2. Types of Grid Lines in MS Excel 2010 –

- Main grid lines: These show the key values of the data ranges. They have usually displayed every 5 or 10 values.

Secondary grid lines: These show the intervals between data ranges. They usually appear after every 1 or 2 values.