Normally, the choice of MPI-libraries does not need to be made explicitly, but is instead chosen by selecting a specific mpi module (see "Software->Modules"). Notice that the MPI module has to match the compiler.
The default setup is to use the Intel compiler suite together with the Intel MPI library.
With the default setup, the program is automatically linking against the SLURM process manager, and it is easy to get optimal performance when running batch jobs (see "Running Batch Jobs"). It is slightly more complicated to run MPI jobs on the frontend machines (see "Running on frontends").
For historical reasons, many other compiler+MPI library combinations exists, but it is not recommended to use them.