A gas diffuses one-third as fast as $\ce{O2}$ at $100^{\circ}\text{C}$. This gas could be:

$ \textbf{(A)}\hspace{.05in}\text{He (M=4)}\qquad\textbf{(B)}\hspace{.05in}\ce{C2H5F}(\text{M=48})$ 
$\qquad\textbf{(C)}\hspace{.05in}\ce{C7H12}\text{(M=96)}\qquad\textbf{(D)}\hspace{.05in}\ce{C5F12}\text{(M=288)}\qquad$
1. **Understanding Graham's Law**: According to Graham's Law of Effusion, the rate of diffusion (or effusion) of a gas is inversely proportional to the square root of its molar mass. Mathematically, this can be expressed as:
   \[
   \frac{r_1}{r_2} = \sqrt{\frac{M_2}{M_1}}
   \]
   where \( r_1 \) and \( r_2 \) are the rates of diffusion of gases 1 and 2, and \( M_1 \) and \( M_2 \) are their respective molar masses.

2. **Given Information**: The problem states that the unknown gas diffuses one-third as fast as \(\ce{O2}\). Therefore, we can write:
   \[
   \frac{r_{\text{unknown}}}{r_{\ce{O2}}} = \frac{1}{3}
   \]

3. **Applying Graham's Law**: Using Graham's Law, we substitute the given ratio into the equation:
   \[
   \frac{1}{3} = \sqrt{\frac{M_{\text{unknown}}}{M_{\ce{O2}}}}
   \]

4. **Solving for the Molar Mass of the Unknown Gas**:
   \[
   \left(\frac{1}{3}\right)^2 = \frac{M_{\text{unknown}}}{M_{\ce{O2}}}
   \]
   \[
   \frac{1}{9} = \frac{M_{\text{unknown}}}{M_{\ce{O2}}}
   \]
   \[
   M_{\text{unknown}} = 9 \cdot M_{\ce{O2}}
   \]

5. **Molar Mass of \(\ce{O2}\)**: The molar mass of \(\ce{O2}\) is:
   \[
   M_{\ce{O2}} = 32 \, \text{g/mol}
   \]

6. **Calculating the Molar Mass of the Unknown Gas**:
   \[
   M_{\text{unknown}} = 9 \cdot 32 = 288 \, \text{g/mol}
   \]

7. **Identifying the Gas**: Among the given options, the gas with a molar mass of 288 g/mol is:
   \[
   \boxed{\textbf{(D)} \ce{C5F12}}
   \]