1. **State the problem:** We need to find the missing fraction $x$ in the equation $$1 - x = \frac{1}{3} + \frac{3}{12}.$$\n\n2. **Combine the fractions on the right side:** First, find a common denominator for $\frac{1}{3}$ and $\frac{3}{12}$. The denominators are 3 and 12, and the least common denominator is 12.\n\nConvert $\frac{1}{3}$ to twelfths: $$\frac{1}{3} = \frac{1 \times 4}{3 \times 4} = \frac{4}{12}.$$\n\nNow add: $$\frac{4}{12} + \frac{3}{12} = \frac{4+3}{12} = \frac{7}{12}.$$\n\n3. **Rewrite the equation:** $$1 - x = \frac{7}{12}.$$\n\n4. **Solve for $x$:** Subtract $\frac{7}{12}$ from both sides: $$x = 1 - \frac{7}{12}.$$\n\n5. **Convert 1 to twelfths:** $$1 = \frac{12}{12}.$$\n\n6. **Subtract:** $$x = \frac{12}{12} - \frac{7}{12} = \frac{12 - 7}{12} = \frac{5}{12}.$$\n\n7. **Answer:** The missing fraction is $\frac{5}{12}$, which matches the option $5/12$.