An example of a sigma bonded organometallic compound is

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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.
\(\underset{\sigma -bond}{\underbrace{R-Mg-X}}\)



The ferrocene compound contains the two \(Fe −(C_2H_5)\) bonds. This Fe is sandwiched between the two cyclopentadienyl ligands. This is a pi bonded system. The plane of the rings is the same distance from the iron atom. Here, the Fe is bonded to the pi bond system thus the ferrocene is π− bonded organometallic compound.
The Cobaltocene, \(Co(C_5H_5)_2\) and Ruthenocene, \(Rh(C_5H_5)_2\) have a similar structure to the ferrocene. So, Cobaltocene and Ruthenocene are also π− bonded organometallic compounds.

Hence, option (b) is correct.