**Answer:**

The C-C bond length is influenced by several factors, including the nature and number of substituents attached to the carbon atoms. In this case, we need to compare the structures given and identify the one with the highest C-C bond length.

**A) CH3_CF3:**
- This compound has a methyl group (CH3) attached to one carbon atom and a trifluoromethyl group (CF3) attached to the other carbon atom.
- The trifluoromethyl group is a highly electron-withdrawing group due to the presence of three electronegative fluorine atoms.
- The electron-withdrawing nature of the trifluoromethyl group will reduce the electron density around the carbon atom it is attached to, leading to a shorter C-C bond length compared to compounds without this group.

**B) FCH2_CH2F:**
- This compound has a fluorine atom (F) attached to each carbon atom.
- Fluorine is also an electronegative atom and will withdraw electron density from the carbon atoms.
- However, in this case, the electron density is being withdrawn by only one electronegative atom per carbon atom, compared to three in the previous compound.
- As a result, the C-C bond length in this compound will be longer than in CH3_CF3.

**C) F2CH__CHF2:**
- This compound has two fluorine atoms (F) attached to one carbon atom and a fluorine atom (F) and a hydrogen atom (H) attached to the other carbon atom.
- Similar to the previous compound, the fluorine atoms will withdraw electron density from their respective carbon atoms.
- However, in this case, there are two fluorine atoms attached to one carbon atom, leading to a stronger electron-withdrawing effect compared to the previous compound.
- As a result, the C-C bond length in this compound will be shorter than in FCH2_CH2F.

**D) CF3__CF3:**
- This compound has a trifluoromethyl group (CF3) attached to each carbon atom.
- As mentioned earlier, the trifluoromethyl group is a highly electron-withdrawing group due to the presence of three electronegative fluorine atoms.
- Since both carbon atoms are attached to the same electron-withdrawing group, the electron density around both carbon atoms will be reduced equally.
- As a result, the C-C bond length in this compound will be shorter than in all the other compounds listed.

**Conclusion:**
- Among the given compounds, the one with the highest C-C bond length is **B) FCH2_CH2F**.