To overcome the memory management issues in multiprogramming and time$-$sharing era, multiple processes are kept to co$-$reside in the main memory. Without proper OS control and hardware collaboration, which problems may pop out:

$1.$ One user process may access to another user process's memory location.

$2.$ A user process may access to OS's memory location.

$3.$ A user process may access to Hardware's memory location.

$4.$ A user process may wait long time for other user process to complete.

$1$ and $2$

$1$ and $2$ and $3$

$1$ and $2$ and $4$

all of them