A: `1 "a.m.u." = 1.66 xx 10^(-24)` gram.
R: Actual mass of one atom of C-12 is equal to `1.99 xx 10^(-23) g`

Options

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

Answer

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

**Assertion (A)**: 1 "a.m.u." = 1.66 x 10^(-24) grams.
**Reason (R)**: The actual mass of one atom of C-12 is equal to 1.99 x 10^(-23) grams.

### Step-by-Step Solution:

1. **Understanding the Definition of AMU**:
- 1 atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.
- Therefore, to find the mass of 1 amu, we need to calculate one twelfth of the mass of a carbon-12 atom.

2. **Calculating the Mass of One Atom of C-12**:
- The mass of one mole of carbon-12 (C-12) is 12 grams.
- One mole contains Avogadro's number of atoms, which is approximately \(6.022 \times 10^{23}\) atoms.
- To find the mass of one atom of C-12, we divide the total mass of one mole by the number of atoms:
\[ \text{Mass of one atom of C-12} = \frac{12 \text{ g}}{6.022 \times 10^{23}} \approx 1.99 \times 10^{-23} \text{ g} \]

3. **Calculating 1 AMU**:
- Now, we can calculate 1 amu using the mass of one atom of C-12:
\[ 1 \text{ amu} = \frac{1}{12} \times \text{Mass of one atom of C-12} \]
- Substituting the mass we calculated:
\[ 1 \text{ amu} = \frac{1}{12} \times 1.99 \times 10^{-23} \text{ g} \approx 1.66 \times 10^{-24} \text{ g} \]

4. **Evaluating the Assertion and Reason**:
- The assertion states that 1 amu = 1.66 x 10^(-24) grams, which we have confirmed to be true.
- The reason states that the actual mass of one atom of C-12 is 1.99 x 10^(-23) grams, which is also confirmed to be true.
- However, while both statements are correct, the reason does not correctly explain the assertion. The assertion is based on the definition of amu, while the reason only provides the mass of one atom of C-12 without linking it to the definition of amu.

### Conclusion:
Both the assertion and reason are correct, but the reason is not the correct explanation of the assertion. Therefore, the correct answer is that both statements are true, but the reason does not explain the assertion.