Q: how do i change my security questions if i can't remember them? why no option?

Forgotten Security Questions/Answers

You need to contact Apple by:

1 - Use the Express lane and start here:

https://expresslane.apple.com

then click More Products and Services>Apple ID>Other Apple ID Topics>Forgotten Apple ID security questions.

or

Apple - Support -form iTunes Store - Contact Us

2 - Call Apple in your country by getting the number from here:
http://support.apple.com/kb/HE57

or          

Apple ID: Contacting Apple for help with Apple ID account security

3 - Use your rescue email address if you set one up

Rescue email address and how to reset Apple ID security questions

For general  information see:

Apple ID: All about Apple ID security questions