Calculate the specific rotation of (R)-2-methylbutanol given the observed rotation, mass, volume, and path length.

Observed rotation ($\alpha$) = $+2.3^\circ$
Mass of (R)-2-methylbutanol = $10.0 \text{ g}$
Volume of solution = $50 \text{ mL}$
Path length of polarimeter tube ($l$) = $20 \text{ cm}$

The specific rotation $[\alpha]$ is calculated using the formula:

where $\alpha$ is the observed rotation, $l$ is the path length in decimeters (dm), and $c$ is the concentration in g/mL.

The specific rotation of (R)-2-methylbutanol is $5.75^\circ \text{ dm}^{-1} (\text{g/mL})^{-1}$.

The calculated specific rotation is 5.75. This matches option (A).

The final answer is $\text{A}$