I think its RMI/IIOP, because nothing is mentioned in the EJB1.1 specs regarding use of JRMP and specified only IIOP. And if you read Cade's chapter 2 page no. 15 ( components of J2EE), it written that EJB container must support following API's: J2SE, JNDI .... , RMI-IIOP..... and it hasnt mentioned JRMP.
So, I think its IIOP only.
Even I found one question somewhere that which is true for EJBs and it has given options like EJB may support RMI-IIOP. EJB has to suuport RMI-IIOP.
So, if we select 1st it means EJB could support something else also as its communication protocol that is JRMP. but I think that is false.
I think the specification says the vendors to use IIOP as the wire protocol. Even if the vendor is using JRMP but still the clients would have to assume that its using IIOP for making the application portable across various application servers.