1. **State the problem:** Aron planted vegetables where $\frac{5}{12}$ were carrots, $\frac{3}{12}$ were cucumbers, and the rest were tomatoes. We need to find the fraction of vegetables that were tomatoes. 2. **Formula and rules:** The total fraction of all vegetables must add up to 1 (or $\frac{12}{12}$). So, the fraction of tomatoes is: $$\text{Fraction of tomatoes} = 1 - \left(\frac{5}{12} + \frac{3}{12}\right)$$ 3. **Calculate the sum of carrots and cucumbers:** $$\frac{5}{12} + \frac{3}{12} = \frac{5+3}{12} = \frac{8}{12}$$ 4. **Subtract to find tomatoes fraction:** $$1 - \frac{8}{12} = \frac{12}{12} - \frac{8}{12} = \frac{12 - 8}{12} = \frac{4}{12}$$ 5. **Simplify the fraction:** $$\frac{4}{12} = \frac{\cancel{4}^1}{\cancel{12}^3} = \frac{1}{3}$$ 6. **Answer:** The fraction of vegetables that were tomatoes is $\frac{1}{3}$. **Final answer: C. $\frac{1}{3}$**