An example of a sigma bonded organometallic compound is:

The correct option is B Grignard's reagent
Grignard's reagent i.e., $R-Mg-X$ is a $\sigma$ -bonded organometallic compound.

The other three organometallic compounds have the metal ($Ru$ in Ruthenocene, $Fe$ in Ferrocene, $Co$ in cobaltocene) sandwiched betweeen two pentene rings. Pentene contains $\pi$ bonds and thus cannot be considered a $\sigma$ bonded organometallic compound.