In a multithreaded environment, which of the following synchronization mechanisms is used to prevent race conditions? 

A semaphore is a synchronization mechanism that is crucial for controlling access to shared resources in a multithreaded environment. It serves as a signaling mechanism that can be used to prevent race conditions, which occur when multiple threads attempt to modify shared data simultaneously, leading to unpredictable outcomes. Semaphores maintain a count of available resources and ensure that only a certain number of threads can access the critical section of code at any given time. When a thread wants to enter a critical section, it checks the semaphore's value; if it is greater than zero, the thread decrements it and proceeds; otherwise, it is blocked until the semaphore is released by another thread. This prevents multiple threads from executing critical sections concurrently, thereby eliminating race conditions and ensuring data consistency. Option B (Fork) - Fork is used to create new processes, not for synchronizing access to shared resources. Option C (Signal) - Signals are used for inter-process communication but do not provide the same level of resource control as semaphores. Option D (Context Switch) - Context switching refers to the operating system's ability to switch between processes and threads, which does not inherently prevent race conditions. Option E (Stack) - A stack is a data structure and does not serve any synchronization purpose in multithreading.