I need help on number 5

A fraction is undefined if its denominator is zero, because then it's saying
"Division by zero !" which is a definite no-no.

Can the denominator of the fraction in #5 ever be zero ?

The denominator is a quadratic polynomial.  Do you know
how to solve (a quadratic polynomial) = 0  ?

This problem is even easier than that, because it gives you some choices.
So you don't even have to solve the quadratic equation.  All you have to do
is check out the choices.
Do you see choice of 'x' values that make the denominator zero ?

Big sloppy hint:  Look at choice-E very very carefully.

Is that enough help ?