Final answer:
A variable resistor or rheostat, represented by a zigzag symbol with an arrow or a diagonal line, would best symbolize an electric fan speed controller in a circuit diagram.
Step-by-step explanation:
The symbol that would best represent an electric fan speed controller in an electric circuit is typically a variable resistor or a rheostat. In circuit diagrams, resistors are denoted by a zigzag symbol, and variable resistors have an arrow or a diagonal line across the zigzag. The arrow or diagonal line indicates that the resistance is adjustable, which aligns with the function of a fan speed controller that varies the current to control the fan speed. Although not directly depicted in the provided reference material, understanding the general representation of components in circuit diagrams allows us to infer the appropriate symbol for a fan speed controller.