A. the toys in the room were new to the children, rather than toys they had previously played with at home:- Old or new toys won't have any effect on sharing, hence this is not the correct answer
B. the observing adult was a parent of the siblings, rather than a stranger or non-relative adult:- The adult will be the parent of a certain number of children, and will be stranger or non-relative adult for all other children. Hence this statement does not strengthen the argument, hence not the correct answer.
C. the siblings were younger and appeared to need help understanding how to use the toys, compared to non-siblings who were adept at using the toys:- in this case the children then should be sharing the toys with non-siblings, as they are adept at using the toys, so this statement actually weakens the argument, Hence not the correct answer.
D. the room had more toys available than usual, making it easier for children to find and share multiple toys:- the number of toys does not have any effect on whether children share toys with siblings or non-siblings. Hence not the correct answer.
E. the adult explicitly encouraged the children to share, regardless of whether the other child was a sibling or a non-sibling:- Here, even after the adult encouraged the children to share irrespective of sibling or non-sibling, children were more likely to share with siblings. Hence this statement strengthens the argument.
Hence the correct answer.