A mobile phone has been stolen. There are 3 suspects P, Q and R. They were questioned knowing that only one of them is guilty. Their responses are as follows:
P: I did not steal. Q stole it.
Q: R did not steal. I did not steal.
R: I did not steal. I do not know who did it.
Who stole the mobile phone?

The correct answer is P.

Let's analyze the situation step-by-step to identify the guilty person among the three suspects: P, Q, and R.
We are given that exactly one person is guilty (stole the mobile phone) and the other two are innocent.

Let's test the hypothesis where P is the thief (guilty):

1. Statements made by P:
- "I did not steal." ⇒ False (since P is guilty).
- "Q stole it." ⇒ False (since P stole it, not Q).

2. Statements made by Q:
- "R did not steal." ⇒ True (since P is the thief, R is innocent).
- "I did not steal." ⇒ True (since P is the thief, Q is innocent).

3. Statements made by R:
- "I did not steal." ⇒ True (since P is the thief, R is innocent).
- "I do not know who did it." ⇒ True (R is innocent and does not know who stole it).

In this scenario, the guilty person (P) lies, while both innocent individuals (Q and R) speak the complete truth. This forms a perfectly consistent logical setup.

Hence, P stole the mobile phone.