A JFET (Junction Field-Effect Transistor) is a three-terminal semiconductor device that can be used as either a voltage-controlled device or a current-controlled device, depending on how it is biased. When properly biased, a JFET behaves as a voltage-controlled current source.

Explanation:

1. JFET Basics:
- A JFET consists of a channel between two terminals called the source and the drain, with a third terminal called the gate controlling the current flow through the channel.
- The channel is formed by a P-N junction, and the type of JFET (N-channel or P-channel) depends on the polarity of the channel material.
- The JFET operates in depletion mode, meaning that the channel is normally conducting current when the gate-source voltage (Vgs) is zero.

2. Voltage-Controlled Current Source:
- When a JFET is biased in the saturation region, it acts as a voltage-controlled current source.
- In this mode, the gate-source voltage (Vgs) is negative, creating a depletion region that narrows the channel, reducing the current flow.
- The current through the JFET (Id) is determined by the gate-source voltage and the JFET's transconductance (gm), which is a measure of its ability to convert a change in gate voltage into a change in drain current.
- As the gate-source voltage increases (in the negative direction), the channel narrows, and the drain current decreases.
- Thus, by varying the gate-source voltage, the current through the JFET can be controlled.

3. Current-Controlled Voltage Source:
- It is worth noting that when a JFET is biased in the cutoff region, it acts as a current-controlled voltage source.
- In this mode, the gate-source voltage is zero or positive, and the JFET does not conduct any current.
- The voltage across the JFET (Vds) is determined by the drain current (Id) and the JFET's output conductance (gd), which is a measure of its ability to convert a change in drain current into a change in drain-source voltage.
- As the drain current increases, the drain-source voltage also increases, resulting in a voltage-controlled response.

Conclusion:
When properly biased, a JFET behaves as a voltage-controlled current source (option C). By varying the gate-source voltage, the current through the JFET can be controlled, making it suitable for applications where a current source with voltage control is required.