Correct answer is C.
Let us use tree diagram to solve this.
Let A be the event define people telling the truth.
A lie detector confirms correctly a person telling lie or truth with probability 0.8.
The tree diagram looks like this,
So, now probability that lie detector confirms true =probability person telling truth and lie detector confirm it true or probability that person telling lie and lie detector confirms true
We have to find given lie detector telling truth, probability that the person actually telling the truth which is