Synchronization primitives are simple software mechanisms provided by a platform (e.g. operating system) to its users for the purposes of supporting thread or process synchronization. They’re usually built using lower level mechanisms (e.g. atomic operations, memory barriers, spinlocks, context switches etc).
Mutex, event, conditional variables and semaphores are all synchronization primitives. So are shared and exclusive locks. Monitor is generally considered a high-level synchronization tool. It’s an object which guarantees mutual exclusion for its methods using other synchronization primitives (usually exclusive locks with condition variables to support waiting and signaling). In some contexts when monitor is used as a building block it is also considered a synchronization primitive.
Critical section is not a synchronization primitive. It’s a part of an execution path that must be protected from concurrent execution in order to maintain some invariants. You need to use some synchronization primitives to protect critical section.