Assertion : Millikan's experiment established that electric charge is quantised.
Reason : From this experiment mass of the electron could not be determined.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that "Millikan's experiment established that electric charge is quantized." This is true. Millikan's oil drop experiment demonstrated that electric charge exists in discrete amounts, specifically as integral multiples of the elementary charge (e).

**Hint:** Recall that quantization means that a quantity can only take on certain discrete values, not continuous ones.

### Step 2: Understand the Reason
The reason given is "From this experiment, the mass of the electron could not be determined." This statement is misleading. While Millikan's experiment primarily focused on measuring the charge of the electron, it also allowed for the calculation of the specific charge (charge-to-mass ratio) of the electron. From this ratio, the mass of the electron can indeed be determined if the charge is known.

**Hint:** Consider how the specific charge (e/m) relates to determining the mass of the electron when the charge is known.

### Step 3: Analyze the Relationship Between Assertion and Reason
Since the assertion is true (Millikan's experiment established the quantization of charge), but the reason provided is false (the mass of the electron can be determined from the experiment), we conclude that the assertion is correct while the reason is incorrect.

**Hint:** Evaluate whether the reason logically supports the assertion. If it does not, then it cannot be considered valid.

### Conclusion
The final conclusion is that the assertion is true, but the reason is false. Therefore, the correct answer would be that the assertion is true, but the reason is false.

**Final Answer:** Assertion is true, Reason is false.