Question is flawed - ribaric acid and erythitol are both optically inactive (meso compounds).
Wasn't written so in my book, so I was confused.
Thanks for the help.
Double check the book to make sure both termini are actually the same (COOH) - if the termini are not identical then it will be chiral, e.g. ribonic acid and ribose are chiral. Assuming you've copied the structure out of the book correctly, there is a mistake in the book.
Can I ask which text it is?