We need the structure of $[Co_2(CO)_8]$.

This is a well-known metal carbonyl dimer. Its usual structure contains:

• one $\mathrm{Co-Co}$ bond
• two bridging carbonyls ($\mu$-CO)
• six terminal carbonyls

We can also check by electron counting.

For each Co atom:

• Cobalt is in group 9, so it contributes $9$ electrons.
• In the bridged dimer form, each Co is bonded to:
  • $3$ terminal CO ligands $\Rightarrow 3 \times 2 = 6$ electrons
  • $2$ bridging CO ligands, each contributing $1$ electron to a given Co in neutral counting $\Rightarrow 2$ electrons
  • one $\mathrm{Co-Co}$ bond $\Rightarrow 1$ electron to each Co

Total per Co: $9 + 6 + 2 + 1 = 18$

So the structure satisfies the $18$-electron rule.

Now evaluate the options:

• A: one $\mathrm{Co-Co}$ bond, six terminal CO and two bridging CO
  This matches the known structure.

• B: one $\mathrm{Co-Co}$ bond, four terminal CO and four bridging CO
  Not the accepted structure for $[Co_2(CO)_8]$.

• C: no $\mathrm{Co-Co}$ bond, six terminal CO and two bridging CO
  Incorrect, because the dimer has a $\mathrm{Co-Co}$ bond.

• D: no $\mathrm{Co-Co}$ bond, four terminal CO and four bridging CO
  Incorrect.

Therefore, the correct option is: $\boxed{\text{A}}$

Comparison with stored correct answer:

• Stored correct answer: A
• Derived answer: A
• They agree.