In this case - http://imgur.com/0wp9rL4
- assuming that qustion is to identify the one which fits with the group on the left it would be a - as it follows the pattern of a white filled large shape, containing two black and one grey smaller shapes.
In practice if you came across a question with multiple possible answers in a test, you'd possibly not notice anyway as you'd just put down the first logical answer you came up with, and stop looking for any more. If you really did find two with valid reasons, then picking what seems the most obvious will probably be the best strategy.
- what are they asking here? Odd one out? It helps if you include the question type as well.