We are asked to find out which of the given expression is in the simplest form.
Let us analyze each of the given expression
Option 1:

It seems that this expression is already in its simplest form.
Option 2:

As you can see, the constant terms can be added together.

So this expression was not in its simplest form.
Option 3:

This expression may be further simplified as

So this expression was not in its simplest form.
Option 4:

This expression may be further simplified as

So this expression was not in its simplest form.
Therefore, now we know for sure that expression 1 was in its simplest form.