Identify the concept being tested

The question asks which metal carbonyl has bridging carbonyl ligands ($\mu$-CO), i.e. a $\mathrm{CO}$ ligand bonded to two metal atoms.

Recall standard structures of the given metal carbonyls

We examine each compound:

A. $\left[\mathrm{Os}_3(\mathrm{CO})_{12}\right]$

• This is a triangular trinuclear cluster.
• In its common structure, all 12 carbonyls are terminal.
• So, it does not typically contain bridging CO ligands.

B. $\left[\mathrm{Ru}_3(\mathrm{CO})_{12}\right]$

• Similar to $\mathrm{Os}_3(\mathrm{CO})_{12}$, this is a trinuclear cluster.
• Its standard structure also has terminal CO ligands only.
• Hence, no bridging CO.

C. $\left[\mathrm{Mn}_2(\mathrm{CO})_{10}\right]$

• This dimer has an $\mathrm{Mn}-\mathrm{Mn}$ bond.
• All 10 CO ligands are terminal.
• So, no bridging carbonyl.

D. $\left[\mathrm{Co}_2(\mathrm{CO})_8\right]$

• This is a well-known case where carbonyl ligands can be bridging.
• The structure of dicobalt octacarbonyl is commonly represented with two bridging CO ligands in the bridged form.
• Therefore, this compound is the correct choice.

Evaluate each option systematically

• A: No bridging CO ✗
• B: No bridging CO ✗
• C: No bridging CO ✗
• D: Bridging CO present ✓

Final answer

The metal carbonyl in which $\mathrm{CO}$ forms a bridge between metal atoms is: $\boxed{\left[\mathrm{Co}_2(\mathrm{CO})_8\right]}$ So, the correct option is D.