In Linux, signal handling can be: a) enabled for asymmetric model of accepting a signal, so that the signal delivery is redirected to another server, for centralized signal handling.
b) enabled for symmetric model of accepting a signal, so that the signal delivery is redirected to another process, for centralized signal handling.
c) enabled for asynchronously catching a signal via a signal handler,
d) enabled for synchronously accepting a signal, which is, to block execution until the signal is delivered to the caller.

d) enabled for synchronously accepting a signal, which is, to block execution until the signal is delivered to the caller.

Explanation:

In Linux, the signal handling process is done through Signal Handlers, which has the ability to function and deliver all the necessary signals synchronously. This means that, when a signal reaches the system, the Signal handler interrupts any execution process that is working at the moment, so that the signal can be efficiently delivered to the caller. However, it is worth mentioning that even if the executions are interrupted, all the current records are saved, which allows the user to return to the activities he was doing, after the signal is delivered.

i got 3,4 and 5 as an answer