Hello Beng,
In order to enter Math expressions in the multiple choice answer, use the TeX filter and look at the following forum postings:
http://moodle.org/mod/forum/discuss.php?d=58615#p266131
For matching questions that need images or substantial equations in the small drop-down menu, you should go in other direction and associate you answers with letter choices. Please look at the following postings:
http://moodle.org/mod/forum/discuss.php?d=69917#p314433
Also look at the "Mathematics Tools" forum for lots of great tips and links.
Have fun!