Sequencer switches are used whenever:

A) a counter function is required.
B) a timer function is required.
C) a time-delay function is required.
D) a repeatable operating pattern is required.

Sequencer switches are mainly used when a repeatable operating pattern is required, which makes option D the best choice. While timers and time-delay functions can be achieved with sequencers, they are not the primary purpose of this type of switch. Similarly, counters are not directly related to sequencer switches.