Step 1: Work with mass per litre:
2.8 g in 0.5 L means 5.6 g per litre. Dividing by the molar mass, $5.6/56 = 0.1$ mol/L.
Step 2: Use hydroxide concentration:
Each KOH gives one $\text{OH}^-$, so $[\text{OH}^-] = 10^{-1}$ M. Then $[\text{H}^+] = \dfrac{10^{-14}}{10^{-1}} = 10^{-13}$ M.
Step 3: Take the negative log:
$\text{pH} = -\log(10^{-13}) = 13$. A strongly basic solution has a pH near 14, so 13 is sensible.
Final Answer:
The hydrogen ion concentration is 1e-13 M, so the pH is 13.
\[ \boxed{\text{(D) }13} \]