Question

which position vs time graph shows an object at rest?

Explanation:

An object at rest has a constant position over time. On a position - time graph, this is represented by a horizontal line as the position does not change with the passage of time.

Answer:

The graph with a horizontal line (assuming one of the four graphs has a horizontal line; without seeing the actual graphs, we know the characteristic of the correct graph is a horizontal line indicating no change in position with time).