Alright here is the question

If Teresa's daughter is my daughter's mother, what am I to Teresa.

Now at first I looked at this and thought. YOU. ARE. TERESA. THIS IS E Z 

Then, I realized.

Wait.

Daughter's mother.

So after more thinking I thought  it was Teresa's S/O

After realizing all the options.

It's the daughter herself

Now here is how you get to that answer

Teresa's daughter, lets name her May

If Teresa's daughter is May

And May is the mother of the daughter of the Narrator

Question is 

Who is the Narrator?

Now this is where I got the S/O thought.

But after looking at the options

You are the 

A: Grandmother

B: Mother

C: Daughter

D: Granddaughter

E: I AM TERESA

Who is the closest to S/O?

The Daughter's mother.

Who is the Daughter's mother?

May, the daughter of Teresa.

So who is May to Teresa??

The daughter.