For Cr{O3} dissolves in aqueous NaOH to give:
A. CrO7^{2 - }
B. CrO4^{2 - }
C. Cr{(OH)3}
D. Cr{(OH)2}

When chromium trioxide \(\text{CrO}_3\) dissolves in aqueous NaOH, it undergoes a reaction with hydroxide ions \(\text{OH}^-\) to form chromate ions \(\text{CrO}_4^{2-}\).

The reaction can be represented as:

\[
\text{CrO}_3 + 2\text{OH}^- \rightarrow \text{CrO}_4^{2-} + \text{H}_2\text{O}
\]

### Explanation:
- **Chromium trioxide** (\(\text{CrO}_3\)) is an acidic oxide of chromium in its highest oxidation state (+6).
- When dissolved in a basic solution such as sodium hydroxide (\(\text{NaOH}\)), it reacts with the \(\text{OH}^-\) ions to form **chromate ions** (\(\text{CrO}_4^{2-}\)).
- **Chromate** is stable in basic medium and is a characteristic product of \(\text{CrO}_3\) reacting with a base.

Thus, the correct answer is:

**B. \(\text{CrO}_4^{2-}\)**