Explanation:

To understand why the correct answer is option 'A', let's first analyze the given information.

Given:
Open-circuit voltage (Voc) = 200 V
Internal resistance (Rin) = 50 Ω

We need to determine the equivalent current source.

Equivalent Current Source:
An equivalent current source is a theoretical concept used to represent an active voltage source. It provides a constant current regardless of the load connected to it. To find the equivalent current source, we need to determine the current it would provide in different load conditions.

To determine the equivalent current source, we can use the concept of Thevenin's theorem.

Thevenin's Theorem:
Thevenin's theorem states that any linear network containing multiple voltage sources and resistors can be replaced by a single voltage source and a single resistor in series.

To find the Thevenin voltage (Vth), we consider the open-circuit voltage. Since the circuit is open, no current flows, and the entire voltage is across the open terminals. Therefore, Vth = Voc = 200 V.

To find the Thevenin resistance (Rth), we need to consider the internal resistance. When we short the open terminals, the entire current flows through the internal resistance. Therefore, Rth = Rin = 50 Ω.

Now that we have determined Vth and Rth, we can represent the voltage source as an equivalent current source in parallel with the Thevenin resistance.

Calculating the Equivalent Current Source:
We know that current (I) = Voltage (V) / Resistance (R).

Using Ohm's law, we can calculate the equivalent current (Ieq) that the current source would provide in different load conditions.

In this case, the equivalent current source is in parallel with the load resistance (RL).

When the load resistance is 50 Ω, the equivalent current (Ieq) can be calculated as:
Ieq = Vth / (Rth + RL)
= 200 V / (50 Ω + 50 Ω)
= 200 V / 100 Ω
= 2 A

Therefore, the equivalent current source is 2 A when the load resistance is 50 Ω.

Since none of the options mention a load resistance of 50 Ω, we need to convert the equivalent current source to a different load resistance.

Using Ohm's law, we can calculate the equivalent current (Ieq) for a load resistance of 25 Ω (50 Ω in parallel).

Ieq = Vth / (Rth + RL)
= 200 V / (50 Ω + 25 Ω)
= 200 V / 75 Ω
= 2.67 A

None of the options match this value.

However, if we consider a load resistance of 100 Ω (50 Ω in series), we can calculate the equivalent current (Ieq) as:

Ieq = Vth / (Rth + RL)
= 200 V / (50 Ω + 100 Ω)
= 200 V / 150 Ω
= 1.33 A

Again, none of the options match this value.

Therefore, the only option that matches the equivalent current source is option 'A': 4A with 50Ω in parallel.