15. In the given circuit, find the value of internal resistance \( r \) of cell in order to have maximum output power in the load resistance circuit. [Given \( R=5 \Omega \) ]
(1) \( 2 \Omega \)
(2) \( 3 \Omega \)
(3) \( 5 \Omega \)
(4) \( 7 \Omega \)