E-Stream Evo 3 27.5 Plus seat post diameter?

[Bully]

I have a BULLS E-Stream Evo 3 27.5 Plus (hardtail) and I'm considering putting a suspension seat post on it. The available diameters are 27.2 mm, 30.9 mm, and 31.6 mm. I don't have calipers to measure. Thanks.

EDIT: The answer is within. As in within the seat post itself, 30.9 mm.

 

[Robie]

I got 31m.m. on my ( seat post ) 2017 Bulls evo f3 27.5 plus full suspension. Break in the rain going riden Manana. Yea haw !!