If we run 5 instances of ThreadA() and 1 instance of ThreadB(), what can be the maximum number of threads active simultaneously in the Critical Section

Only one

Explanation:

When dealing with a critical section, only one thread may be active in it a single moment. If more than one thread is trying to gain access then only one will be allowed while the other would be placed in a queue to access it next. This is mainly done in order to prevent two threads/processes from accessing/manipulating the same variable or data at the same time which can cause program breaking errors to occur.

that without the operating systems you would have to control and synchronize computer hardware.