Ricart & Agrawala's mutual exclusion algorithm differs from Lamport's because it;

a$)$ Uses more messages than Lamport's since each message needs to be acknowledged

b$)$ Uses fewer messages than Lamport's since a system does not reply until it agrees to grant access to a resource.

c$)$ Does not require the use of Lamport timestamps

d$)$ Requires a system to contact all group members to request access to a resource.