What kind of concrete would you use in a retention wall? c10, c20, c30, c40, c50

Concrete type is not important in the retaining wall study .. because it's about bending resistance .. and that's related to the used steel, not concrete ..

I think C30 is sufficient anyway ..

Thankyou 

Basing on type of retaining (retention) wall, i.e. gravity or RCC. If design is based on gravity, any grade of concrete mentioned, can be used. The principle behind gravity is the wall retains by self weight .

In RCC retaining wall we have to use minimum C20 or higher. As we use higher grade concrete, sections can be made thin and hence materials usage will be reduced.

That depends completely on the design/use of the retention wall , also on environment, but a minimum of C30-C30/37 I would prefer.

c50