"Can I get some rice?"
Wa is for indicating what you are talking about, the topic. O is put after the direct object (in the phrase "I eat the apple" apple is the direct object). Sometimes they overlap ("speaking of the apple, i ate it" the apple is both the topic and the direct object) but not in this case.
Minor complaint - it marked this wrong because my keyboard changed it to ご飯 Instead of ごはん（I know that 飯 means rice from learning Mandarin so assumed it would be the same here）. I checked and it seems that this is the right Kanji, if so could this be fixed so that ご飯 is accepted？Or if I'm wrong about this then no worries.