Assertion: Force of bouyancy due to atmosphere on a small body is almost zero(or negligible).
Reason: If a body is completely submerged in a fluid, then buoyant force is zero.

Options

A. (a) If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. (b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that the "Force of buoyancy due to atmosphere on a small body is almost zero (or negligible)."

- **Understanding Buoyancy**: The buoyant force (or force of buoyancy) on an object submerged in a fluid is given by the equation:
\[ F_b = V \cdot \rho \cdot g \]
where \( V \) is the volume of the object, \( \rho \) is the density of the fluid, and \( g \) is the acceleration due to gravity.

- **Small Body in Air**: For a small body in the atmosphere (air), the volume \( V \) is small, and the density of air (\( \rho \)) is also very low (approximately \( 1.2 \, \text{kg/m}^3 \)). Therefore, the product \( V \cdot \rho \) will be very small, making the buoyant force negligible.

Thus, the assertion is **true**.

### Step 2: Analyze the Reason
The reason states that "If a body is completely submerged in a fluid, then buoyant force is zero."

- **Understanding Complete Submersion**: When a body is completely submerged in a fluid, it experiences a buoyant force equal to the weight of the fluid displaced by it. According to Archimedes' principle, the buoyant force acts upward and is equal to the weight of the fluid that the body displaces.

- **Conclusion on Reason**: Since the buoyant force is not zero when a body is completely submerged, the reason is **false**.

### Step 3: Conclusion
Now we can conclude:
- The assertion is true.
- The reason is false.

Thus, the correct answer is **Option C**: "Assertion is true, but the reason is false."

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