Explanation:
A Wheatstone bridge is a circuit used to measure an unknown resistance by balancing two legs of a bridge circuit, one with a known resistance and the other with an unknown resistance. The bridge is balanced when the voltage difference between the two midpoints is zero, indicating that the resistances are equal.

In this case, the resistances are given as 120Ω, 150Ω, and 150Ω in a clockwise direction. We need to find the value of the fourth resistance for null balance.

Steps to solve the problem:

1. Understand the concept of a Wheatstone bridge.
A Wheatstone bridge consists of four resistances connected in a diamond shape with a voltage source and a galvanometer. The two top resistances (R1 and R2) are known resistances, and the two bottom resistances (R3 and R4) are unknown resistances. The bridge is balanced when the voltage difference between the two midpoints is zero.

2. Analyze the given circuit.
In the given circuit, the known resistances are 120Ω, 150Ω, and 150Ω in a clockwise direction. The fourth resistance is unknown (let's call it Rx). To find the value of Rx, we need to balance the bridge by adjusting the resistance Rx.

3. Apply the balance condition.
For the bridge to be balanced, the ratio of the resistances on one side of the bridge should be equal to the ratio of the resistances on the other side. Mathematically, this can be expressed as:
R1/R2 = R3/Rx

4. Substitute the given values.
In this case, R1 = 120Ω, R2 = 150Ω, and R3 = 150Ω. We need to find the value of Rx.

5. Solve the equation.
Substituting the given values into the balance condition equation, we have:
120/150 = 150/Rx

6. Cross-multiply and solve for Rx.
Cross-multiplying the equation, we get:
120Rx = 150 * 150

Dividing both sides of the equation by 120, we get:
Rx = (150 * 150) / 120

Calculating the value, we have:
Rx = 187.5Ω

7. Check the options.
From the given options, the closest value to 187.5Ω is 120Ω (option B).

Therefore, the correct answer is option B, 120Ω.