C

Step 1: Let's analyze the assertion. The assertion states: 'If the sum of the two unit vectors is also a unit vector, then the magnitude of their difference is root of three.'
For two unit vectors, let them be represented as \\mathbf{a} and \\mathbf{b}, where |\\mathbf{a}| = |\\mathbf{b}| = 1. The condition that their sum is a unit vector means that |\\mathbf{a} + \\mathbf{b}| = 1.
Step 2: Using the formula for the magnitude of the sum of two vectors, we have:
|\\mathbf{a} + \\mathbf{b}|^2 = |\\mathbf{a}|^2 + |\\mathbf{b}|^2 + 2 |\\mathbf{a}| |\\mathbf{b}| \\cos(\\theta) = 1.
Since |\\mathbf{a}|^2 = 1 and |\\mathbf{b}|^2 = 1, we get:
1 + 1 + 2 \cdot 1 \cdot 1 \cdot \cos(\\theta) = 1 \implies 2 + 2 \cos(\\theta) = 1 \implies 2 \cos(\\theta) = -1 \implies \cos(\\theta) = -\frac{1}{2}.
The angle between the vectors is 120 degrees. Now, the magnitude of their difference can be calculated using:
|\\mathbf{a} - \\mathbf{b}|^2 = |\\mathbf{a}|^2 + |\\mathbf{b}|^2 - 2 |\\mathbf{a}| |\\mathbf{b}| \cos(\\theta) = 1 + 1 - 2 \cdot 1 \cdot 1 \cdot ( -\frac{1}{2}) = 1 + 1 + 1 = 3 \implies |\\mathbf{a} - \\mathbf{b}| = \sqrt{3}.
Hence, the assertion is true.
Step 3: Now, let's analyze the reason. The reason given is: 'To find the resultant of two vectors, we use the square law.' While this is a commonly used method to find the resultant of two vectors, the specific context of the assertion is about the conditions under which the sum maintains unit status and deriving differences using cosine laws, hence the reason is a general statement not directly explaining the assertion.
Therefore, the assertion is true, but the reason is false.
Conclusion: The correct choice is option C.