a: I saw an apple, an orange, and a pear.
b: I saw an apple, an orange and a pear.
Your test shows "b" as correct. This is incorrect. For Americans, "a" is usually preferred. However, this is a debated topic: http://en.wikipedia.org/wiki/Serial_comma
Due to the debate on the issue, you might want to simply call both right or both wrong. It's definitely incorrect to single either out as wrong.