Answer: b). matched-samples design
Explanation:
Matched-samples design  is the design in which matching mechanism in used  samples. Participants share matched samples every characteristic expect for the trait that is being researched or experimented.
According to the question, matched pair of brother and sisters are involved in research for describing positivity rate in family in childhood.Thus, they have matched characteristics for finding positivity rate.
Other options are incorrect because repeated measure and both the measures together are not depicted through design in case.Thus, the correct option is option(b).
