Mr. Harris attaches a full tank of propane to his grill. Then he turns on the grill and cooks food for a barbecue. Which graph shows the relationship between the amount of propane in the tank and time?

The correct answer is:

The graph shows the relationship between the amount of propane in the tank and time.

Correct option is (b).
At time 0 minute the tank was full
Then Mr Harris coooked which will bring down the level of propane in the tank with passage of time and clearly in option (b) the graph is dowanward means with the passage of time the level of propane is getting down