The focus of the text is that the colour of the interrogation room can help the police to be more successful during interrogation. However, there is no guarantee that it will make an interrogation more successful.
A – Correct
An interrogation is successful when truth is extracted from people. The first sentence “A Welsh police station….intention of improving the success of interrogation” suggests that colour which can help people to be more at ease can improve success in interrogation.
B – Wrong
It is not implied that psychologist have full knowledge of what happens in an interrogation but the text just says that psychologists know which colours help people to be more relaxed. More relaxed people will tend to speak more truthful.
C – Wrong
Colour psychology can aid the interrogation process but it is in no way implied that it is the best way.
D – Wrong
This is not an assumption.
E – Wrong
The text clearly does not assume that good interrogation technique is less useful than colour psychology. It only suggests that colour psychology can help people to be more relaxed.