The investigating team had authentic information that S never lies.
from this we can be assured that $S$ is telling truth.
Statement of S: Only one of us is telling the truth.
if $S$ is telling truth and he is telling only one of us is telling truth that means he is telling for everyone including him also. so, if only one person is telling truth and we know $S$ is telling truth that means only $S$ is telling truth. so, rest all are lying.
Statement of P: R has copied in the exam. False
Statement of Q: S has copied in the exam. False
Statement of R: P did not copy in the exam. False
Statement of S: Only one of us is telling the truth. True
Statement of T: R is telling the truth. False
so, new statements will look like this,
Statement of P: R has not copied in the exam.
Statement of Q: S has not copied in the exam.
Statement of R: P copied in the exam.
Statement of S: Only one of us is telling the truth.
Statement of T: R is not telling the truth.
information we got from statement of T is redundant now, as we already knew before that R was not telling truth and rest all statements support the fact that P has copied in the exam.
Based on the above statements, we can make the pictorial representation.
$\qquad \begin{array}{ccccc} \underbrace{\text{P}}_{\text{R}\;\Large{\checkmark}} & \underbrace{\text{Q}}_{\text{S}\;\Large{\checkmark}} & \underbrace{\text{R}}_{\text{P}\Large{\times}} & \underbrace{\text{S}}_{\text{Only one is true}} & \underbrace{\text{T}}_{\text{R is true}} \\\hline \underbrace{\underbrace{\text{P}}_{\text{R}\;\Large{\checkmark}}}_{{\color{Red}{\text{False}}}} & \underbrace{\underbrace{\text{Q}}_{\text{S}\;\Large{\checkmark}}}_{{\color{Red}{\text{False}}}} & \underbrace{\underbrace{\text{R}}_{\text{P}\Large{\times}}}_{{\color{Red}{\text{False}}}} &\underbrace{\underbrace{\text{S}}_{\text{Only one is true}}}_{{\color{Green}{\text{Always True}}}} & \underbrace{\underbrace{\text{T}}_{\text{R is true}}}_{{\color{Lime}{\text{True}}}} \\\hline \underbrace{\text{P}}_{\text{R}\;\Large{\times}} & \underbrace{\text{Q}}_{\text{S}\;\Large{\times}} & \underbrace{\text{R}}_{\text{P}\;\Large{\checkmark}} & & \end{array}$
$\therefore$ The person who has copied in the exam is ${\color{Blue}{\text{P}.}}$
