What counter is needed to turn on a service soon light after 100 cars leave a car wash?

The counter needed to turn on a service soon light after 100 cars leave a car wash is a count-up counter, which is represented by CTU (Count Up). The CTU counter is designed to increment its count each time a specified event occurs, which in this scenario would be each car that leaves the car wash.

When a car exits, the count increases by one. Once the CTU count reaches 100, the system can then trigger the service soon light to alert personnel that a specific threshold has been met—indicating that 100 cars have successfully been washed. This responsive action enables operators to manage car wash operations effectively based on usage metrics.

In contrast, the other options serve different functions that do not align with the requirement of tracking cumulative counts of cars. CTD (Count Down) would decrement a count, which is not suitable for this scenario, while RES (Reset) is used to reset a counter, rather than count events. TON (Timer On Delay) is designed to measure elapsed time before triggering an output, which doesn't directly correlate with counting the number of cars. Thus, the CTU counter is the appropriate choice for this task.