which of the following Expressions is in the simplest form

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:
[tex]8m+4n-5mn+7[/tex]It seems that this expression is already in its simplest form.
Option 2:
[tex]15bd+5-2b+3d+23[/tex]As you can see, the constant terms can be added together.
[tex]\begin{gathered} 15bd-2b+3d+5+23 \\ 15bd-2b+3d+28 \end{gathered}[/tex]So this expression was not in its simplest form.
Option 3:
[tex]xy+6xy^2-7+12xy[/tex]This expression may be further simplified as
[tex]\begin{gathered} xy+6xy^2+12xy-7 \\ x(y+6y^2+12y)-7 \\ x(6y^2+13y)-7 \end{gathered}[/tex]So this expression was not in its simplest form.
Option 4:
[tex]35-a^2-ab^2+ab-2ab^2[/tex]This expression may be further simplified as
[tex]\begin{gathered} 35-a^2-ab^2+ab-2ab^2 \\ 35-a^2+ab-3ab^2 \end{gathered}[/tex]So this expression was not in its simplest form.
Therefore, now we know for sure that expression 1 was in its simplest form.