G, from a regulatory aspect (ICAO Annex 6) and National Authorities Ops Reqs; all that they required is to have the appropriate level of TAWS fitted.
Technical requirements are in AC25-23 which outlines the manner in which TAWS should work, and also in TSO's (TSO 151b?).
My knowledge is dated, but I would be very surprised if any TAWS ‘A’ would be certifiable without Rad Alt.
TAWS ‘B’ might be with the use of GPS, see CAA Letter of Consultation 6 Nov 2002, but most documentation suggest that Geometric Altitude is not accurate enough.