The correct answer is D. Any amount of time over an hour and a half would cost $10.
f (t), when t is a value between 0 and 30
The cost is US$ 0 for the first 30 minutes
f (t), when t is a value between 30 and 90
The cost is US$ 5 if the connection takes between 30 and 90 minutes
f (t), when t is a value greater than 90
The cost is US$ 10 if the connection takes more than 90 minutes
According to these costs, statements A, B and C are incorrect. The connection doesn't cost US$ 5 per hour like statement A affirms, the cost of the connection isn't US$ 5 per minute after the first 30 minutes free as statement B affirms and neither it costs US$ 10 for every 90 minutes of connection, as statement C affirms. The only one that is correct is D, because any amount of time greater than 90 minutes actually costs US$ 10.