First off I would stress that architecture and process are more
important than which of those 3 languages you choose, i.e. good
testing (I prefer test driven), continuous integration, and a
solid architecture that you are confident will provide the
reliability, correctness and uptime that you require.
Having said that I would then personally be conservative and
choose to standardise on C# for its maturity, expressiveness and
great tooling. It also has a good ecosystem (libraries etc.)
which will prove very useful in business related tasks.
D has better expressiveness and probably would run faster but
given all the other factors I would be concerned right now about
its slight lack of maturity and under-developed ecosystem.
