When reading diagrams like these, you have to know that the dots shown with a line are representative of the poles in a switch. The throw refers to how many “on” positions the switch has.
Diagram A shows a single pole, triple throw switch.
Diagram B shows a double pole, double throw switch.
Diagram C shows a three-pole, double throw switch.
And Diagram D, the correct answer, shows a triple pole, single throw switch.