An example of a sigma bonded organometallic compound is

The correct option is B grignard's reagent

The organometallic compounds having sigma bond between carbon and metal are sigma bonded organomettalic. An example of a sigma bonded organometallic compound is Grignard's reagent.
$\underset{\sigma -bond}{\underset{}{R-Mg-x}}$
Whereas, ruthenocene, ferrocene and cobaltocene are not sigma bonded organometallic compound.