Given data:
- Transconductance (gm) of the JFET = 4 ms
- External drain resistance (RD) = 1.5 ohm

We need to find the ideal voltage gain.

The ideal voltage gain (Av) of a JFET amplifier can be calculated using the formula:

Av = -gm * RD

Explanation:
1. Transconductance (gm):
The transconductance (gm) is a measure of how much the drain current (ID) changes for a given change in the gate-source voltage (VGS).
It is given by the formula:

gm = ΔID / ΔVGS

Here, gm is in siemens (S), ID is in amperes (A), and VGS is in volts (V).

2. External drain resistance (RD):
The external drain resistance (RD) is the resistance connected to the drain terminal of the JFET. It is used to provide the load for the amplifier.

3. Ideal voltage gain (Av):
The ideal voltage gain (Av) is the ratio of the change in output voltage (ΔVout) to the change in input voltage (ΔVin) in the amplifier.
It can be calculated using the formula:

Av = ΔVout / ΔVin

In a JFET amplifier, the ideal voltage gain is given by:

Av = -gm * RD

Explanation of the formula:
- The negative sign in the formula is due to the fact that the JFET is a voltage-controlled device and the output voltage is out of phase with the input voltage.
- The transconductance (gm) determines the slope of the transfer characteristic curve of the JFET.
- The external drain resistance (RD) determines the load on the JFET and thus affects the gain.

In this case, the transconductance (gm) is given as 4 ms and the external drain resistance (RD) is given as 1.5 ohm.
Therefore, the ideal voltage gain (Av) can be calculated as:

Av = -4 ms * 1.5 ohm = -6

The negative sign indicates that the output voltage will be out of phase with the input voltage.
Hence, the ideal voltage gain is 6 (option C).