(A) : The most common oxidation state of lanthanides is `+3`.
(R): Lanthanides have three electrons in their outermost shell. 

Options

A. Both A & R are true, R is the correct explanation of A
B. Both A & R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Answer

To solve the question regarding the assertion and reason about the oxidation state of lanthanides, let's break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "The most common oxidation state of lanthanides is +3."

- **Explanation**: Lanthanides are a series of 15 elements from cerium (Ce) to lutetium (Lu) in the periodic table. The most common oxidation state for these elements is indeed +3. This is due to the fact that they tend to lose three electrons during chemical reactions.

### Step 2: Analyze the Reason
The reason states that "Lanthanides have three electrons in their outermost shell."

- **Explanation**: To analyze this statement, we need to look at the electronic configuration of lanthanides. The general electronic configuration for lanthanides is:
\[ [Xe] 4f^{1-14} 5d^{0-1} 6s^2 \]
In this configuration, the outermost shell (6s) has 2 electrons, and the 4f and 5d orbitals contribute additional electrons. Therefore, the statement that they have "three electrons in their outermost shell" is incorrect.

### Step 3: Conclusion
Based on the analysis:
- The assertion is **true** because the most common oxidation state of lanthanides is indeed +3.
- The reason is **false** because lanthanides do not have three electrons in their outermost shell; they have only 2 in the 6s orbital.

### Final Answer
The correct answer is that the assertion is true, but the reason is false.