Assertion (A): If log x = $\dfrac{\text{ log } 8}{\text{ log } 0.25}$, then x = -6.

Reason (R): If log _a b = log _a c, then b = c.

1. Assertion (A) is true, Reason (R) is false.

2. Assertion (A) is false, Reason (R) is true.

3. Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct reason for Assertion (A).

4. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct reason (or explanation) for Assertion (A).

According to reason,

If log _a b = log _a c, then b = c.

If the logarithms of two numbers are equal, and they share the same base, then the numbers themselves must be equal.

∴ Reason (R) is true.

According to assertion,

$\Rightarrow \text{log x} = \dfrac{\text{ log } 8}{\text{ log } 0.25}\\[1em] \Rightarrow \text{log x} = \dfrac{\text{ log } 8}{\text{ log } \Big(\dfrac{25}{100}\Big)}\\[1em] \Rightarrow \text{log x} = \dfrac{\text{ log } 2^3}{\text{ log } \Big(\dfrac{1}{4}\Big)}\\[1em] \Rightarrow \text{log x} = \dfrac{\text{ log } 2^3}{\text{ log } \Big(\dfrac{1}{2^2}\Big)}\\[1em] \Rightarrow \text{log x} = \dfrac{\text{ log } 2^3}{\text{ log } 2^{-2}}\\[1em] \Rightarrow \text{log x} = \dfrac{3\text{ log } 2}{-2\text{ log } 2}\\[1em] \Rightarrow \text{log x} = \dfrac{-3}{2} \\[1em] \Rightarrow x = (10)^{-\dfrac{3}{2}}.$

∴ Assertion (A) is false.

∴ Assertion (A) is false, Reason (R) is true.

Hence, option 2 is the correct option.