TransactionScopeAsyncFlowOption for the TransactionScope was added in .NET 4.5.1 and seems has no implementation on Mono(3.8.0).
So I'm looking for a correct workaround or maybe some kind of a "polyfill".
P.S.: Seems mono development process is closed for any feature requests. So I didn't find a way to report about this feature.
Asked for a feature on Xamarin's issue tracker quite long ago. Seems it's not a priority for Xamarin's team.