Explanation:

- Option 1: upon

- "Upon" usually means on the surface of something. It doesn’t fit well here since it would suggest leading someone to stand on the lawn, not across it.

- Option 2: across

- "Across" implies moving from one side to the other. It fits perfectly here as it describes being led from one side of the lawn to the other to reach a building.

The correct answer is option 2: across.

- Option 3: along

- "Along" suggests moving in line with something, like along a path or a street. It indicates direction but not necessarily crossing an area.

- Option 4: about

- "About" typically means around or here and there. It doesn't work well in this context since it doesn’t convey moving from one point to another.