Given:
A millivoltmeter has a range of 25 millivolts.
We need to convert the millivoltmeter into an ammeter with a range of 25 amperes.

To find:
The value of the necessary shunt (in ohms).

Solution:
To convert the millivoltmeter into an ammeter, we need to connect a shunt resistor in parallel to the millivoltmeter. The shunt resistor will divert most of the current, allowing only a small fraction of the current to pass through the millivoltmeter.

Step 1: Conversion of millivolts to amperes:
The range of the millivoltmeter is 25 millivolts. To convert this range into amperes, we need to use Ohm's law, which states that voltage equals current multiplied by resistance (V = IR).

Since the millivoltmeter has a range of 25 millivolts, we can write the equation as:
25 millivolts = I (current) * R (resistance)

To convert millivolts to volts, we divide by 1000:
25 millivolts = 0.025 volts

Therefore, the equation becomes:
0.025 volts = I (current) * R (resistance)

Step 2: Determining the necessary shunt resistance:
We want to convert the millivoltmeter into an ammeter with a range of 25 amperes. We can write the equation as:
25 amperes = I (current passing through the millivoltmeter) + I (current passing through the shunt resistor)

Since most of the current will pass through the shunt resistor, we can assume that the current passing through the millivoltmeter is negligible. Therefore, the equation becomes:
25 amperes = I (current passing through the shunt resistor)

Using Ohm's law, we can write this equation as:
25 amperes = 0.025 volts / R (shunt resistance)

Simplifying the equation, we get:
R (shunt resistance) = 0.025 volts / 25 amperes

Simplifying further, we get:
R (shunt resistance) = 0.001 ohms

Therefore, the value of the necessary shunt resistance is 0.001 ohms.

Hence, the correct answer is option 'A' (0.001).