In each of the following questions a Statement of Assertion (A ) is given followed by a corresponding statement of Reason (R ) Just below it . Of the statement mark the correct answer as
Assertion. The element with electronic configuration `[Xe]^(54) 4f^(1) 5d^(1) 6s^(2) ` is d- block element
Reason . the last electron enters the d-orbital.

Explanation

To solve the question step by step, we need to analyze both the Assertion (A) and the Reason (R) provided.

### Step 1: Understand the Assertion
The assertion states that the element with the electronic configuration `[Xe]^(54) 4f^(1) 5d^(1) 6s^(2)` is a d-block element.

### Step 2: Analyze the Electronic Configuration
The electronic configuration given is:
- `[Xe]` indicates that the element has 54 electrons (the same as Xenon).
- The `4f^(1)` indicates that there is one electron in the 4f subshell.
- The `5d^(1)` indicates that there is one electron in the 5d subshell.
- The `6s^(2)` indicates that there are two electrons in the 6s subshell.

### Step 3: Determine the Last Electron's Subshell
To determine if the element is a d-block element, we need to identify where the last electron enters:
- The last electron enters the 5d subshell (since `5d^(1)` is the last in the configuration).

### Step 4: Identify the Block of the Element
According to the Aufbau principle, the order of filling is based on the increasing energy levels. The d-block elements are characterized by the filling of d orbitals. However, the presence of the 4f electron means that the element is actually an f-block element (specifically a lanthanide or actinide).

### Step 5: Conclusion on Assertion
The assertion is **false** because, despite the last electron entering the d orbital, the presence of the 4f electron means that this element is classified as an f-block element, not a d-block element.

### Step 6: Analyze the Reason
The reason states that the last electron enters the d-orbital. This statement is **true** because the last electron indeed enters the 5d subshell.

### Final Answer
- Assertion (A): False
- Reason (R): True

Thus, the correct answer is that the assertion is false and the reason is true.