When using branching, make a question "Required" based on the response to a previous question.
1) Do you have a smartphone (required)
2) Which operating system does your smartphone have (only required if the answer to 1 is a)
c) Windows Phone
3) Totally unrelated question
It's possible to do it with Google forms.
For an Evaluate question, if you have less than 2 stars you have a mandatory question Following.
This is already part of the product. You can simply set up (from your example) answer 1a goes to next question, answer 1b goes to question 3, and question 2 is required. When the branching causes question 2 to be skipped, it doesn't matter that question 2 is required because the user will never see it and will never have to answer it.