Assertion (A) : The pairing of electrons in the orbitals of a particular subshell does not occur until all the orbitals of the subshell are singly occupied.
Reason (R) : Singly occupied orbitals must have the electrons with parallel spins.

Options

A. Both A and R true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To analyze the given assertion and reason, we can break it down into steps:

### Step-by-Step Solution:

**Step 1: Understanding the Assertion (A)**
The assertion states that the pairing of electrons in the orbitals of a particular subshell does not occur until all the orbitals of the subshell are singly occupied. This is a reference to Hund's Rule, which states that electrons will fill degenerate orbitals (orbitals of the same energy) singly before pairing up.

**Step 2: Analyzing the Reason (R)**
The reason states that singly occupied orbitals must have electrons with parallel spins. This is also true and is a consequence of Hund's Rule. When electrons occupy the same subshell, they will first fill each orbital singly, and when they do so, they will have parallel spins (same spin direction).

**Step 3: Evaluating the Truth of Assertion and Reason**
Both the assertion and the reason are true statements:
- Assertion (A) is true because of Hund's Rule.
- Reason (R) is true because it describes the behavior of electrons in singly occupied orbitals.

**Step 4: Determining the Relationship Between A and R**
While both statements are true, the reason provided does not correctly explain the assertion. The assertion is based on Hund's Rule, while the reason discusses the spin of the electrons, which is a separate but related concept.

### Conclusion:
Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion. Therefore, the correct option is that both A and R are true, but R is not the correct explanation of A.

### Final Answer:
Both A and R are true, but R is not the correct explanation of A.

---