Final answer:
A semaphore is a synchronization tool used to control access to common resources in a concurrent system, preventing race conditions.
Step-by-step explanation:
The correct answer to the question of which one of the following is a synchronization tool is c) semaphore. A semaphore is a synchronization primitive that is used to control access to a common resource in a concurrent system such as a multitasking operating system. It helps to prevent race conditions when multiple threads or processes need to access the shared resource.
Options a) thread, b) pipe, and d) socket are not synchronization tools. A thread is a unit of execution within a process, a pipe is a method of inter-process communication, and a socket is an endpoint for sending or receiving data across a computer network.