a process: because very little memory copying is required (just the thread stack), threads are faster to start than processes. To start a process, the whole process area must be duplicated for the new process copy to start.

Which is more efficient thread or process?

The process is less efficient in terms of communication. Thread is more efficient in terms of communication.

What is an advantage of using threads vs processes?

Fault-tolerance and scalability are the main advantages of using Processes vs. Threads. A system that relies on shared memory or some other kind of technology that is only available when using threads, will be useless when you want to run the system on multiple machines.