Which type of resistor has a value that cannot be changed?

The correct answer is a fixed resistor. A fixed resistor has a specific and unchangeable resistance value that remains constant regardless of the voltage or current passing through it. This stability is key in many electronic circuits where a precise resistance is required for proper functioning.

In contrast, a variable resistor allows for adjustments to be made to its resistance value, making it suitable for applications where tuning is needed. Potentiometers are a type of variable resistor specifically designed to adjust voltage levels, further emphasizing their ability to change resistance. Thermistors are temperature-sensitive resistors that change their resistance in response to temperature variations, meaning their resistance value is not fixed.

Thus, the primary distinguishing characteristic of a fixed resistor is its consistent resistance, which does not change in response to external conditions or user adjustments.