First line shows in passage that there is no difference between human and ape in psychological studies .
While 2nd line says it needs more studies to be conclude so that means there is a difference. Which supported by option D .
Option A This passage have no point about highly developed psychological capacities .
Option B This passage have no point about studies on mammals
Option C , which says that objectivity or correct judgement is nearly impossible in psychological studies due to lack of evidence . But Question says about it is inadequate not impossible.