Dear Anggie,
Please check our Reference manual for information about modelling undrained behaviour. This is chapter explains the fact that the undrained shear strength su can be considered as an input parameter for Undrained B and C options.
Note that in general in PLAXIS the undrained shear strength is computed based on the performed analysis and depends on the constitutive model. This approach is used with Undrained A option. The resulting mobilised shear strength must be checked against known data.