But the answer is 200j

Step 1: Write the formula for current in a circuit.
$I = \frac{V}{Z}$

Step 2: Identify the values with units. Voltage $V = 100\,\mathrm{V}$, capacitive impedance $Z = -j\,0.5\,\Omega$.
Substitute into the formula:
$I = \frac{100\,V}{-j\,0.5\,\Omega}$

Step 3: Simplify the denominator. Multiply numerator and denominator by the conjugate of the denominator or rewrite:
$\frac{1}{-j} = \frac{1}{-j} \cdot \frac{j}{j} = \frac{j}{(-j)j} = \frac{j}{-j^2} = \frac{j}{1} = j$
So,
$I = \frac{100\,V}{0.5\,\Omega} \cdot j = 200\,A \cdot j$

Final answer: $\boxed{200j\,A}$