If Light 1 is Purple we know that Light 2 will be yellow according to the conditions. If Light 2 is Yellow we know that Light 3 will be Green. The reason being that Light 3 can't be Purple or Yellow because that would make Light 2 Purple according to the other condition. However, as we know by choosing Light 1 as Purple, Light 2 has to be Yellow. That is the only answer choice that would give you a strict sequence.