Step 1: Differentiate implicitly:
\(2yy'-y-xy'=0\), so \(y'=\dfrac{y}{2y-x}\).
Step 2: Vertical tangent condition:
The slope is infinite when the denominator is zero: \(2y-x=0\), so \(x=2y\).
Step 3: Use the curve equation:
Substitute \(x=2y\): \(y^2-2y^2+9=0\), so \(y^2=9\) and \(y=\pm3\). Option D.
Step 4: Why the other options are wrong.
At \(y=0\) the curve equation gives \(9=0\), impossible. The values \(\pm\sqrt3\) and \(\tfrac12\) do not satisfy \(y^2=9\).
Final Answer:
The tangent is vertical when y = plus or minus 3.
\[ \boxed{\text{(D) }y=\pm3} \]