Hi JKS,
Answer choice (D) states that these two speakers agree that we know
something about the effects of depictions of violence on audiences. To confirm this, we need to see if both speakers point to an effect of the depictions of violence as a fact.
Liang states that it is true that watching violence sometimes increases levels of aggression in viewers She would thus agree that we know something about the effects of watching depictions of violence.
Sarah claims that watching dramas with violent characters has the effect of allowing viewers to vicariously experience some emotions and thus purge themselves of those emotions.
Liang and Sarah might not agree on any specific effect that violence has on audiences, but we don't need to prove that they do. They would both claim that they understand something about the effects of watching violence on audiences, which is all that answer choice (D) requires.
Let me know if this helps!