(A) Total events / User type
(B) Sessions / Source
(C) Sessions / Bounce rate
(D) Avg. time on page / device category
The invalid Metric-Dimension combination is the Sessions/Bounce rate because they have different Scope. If the Metric is Bounce Rate then the Correct Dimension will be Source/Medium.
This question is a part of the Google Analytics Individual Qualification. You can get all the answers with detailed explanation for this exam in our Google Analytics Individual Qualification Exam Answers page.