You just chose the right answer. Dividing the carbonyls among the osmium atomes gives 4 carbonyls per osmium. With the 8 valence electrons from metal this brings the total to 16 electrons. The only remaining place to get electrons is from metal-metal bonds. Therefor, each osmium needs 2 Os-Os bonds. The only possible shape where each osmium is bonded to 2 other osmiums is a triangle.

Os             = 8e- 4 x COT    = 8e- 2 x Os-Os = 2e-                     18e-

But there is another alternative structure that also obeys The Eighteen Electron Rule. Have a look at it.