Assume battery is 3.0 V.
If S is broken , only P and Q lights up. Pd across S is 1.5 V and Pd across R is zero.
If R is broken , only P and Q lights up. Pd across R is 1.5V and Pd across S is zero.
If Q is broken , R , S and P lights up . Pd across Q is 2.0V and Pd across each bulb is 1.0V.
If P is broken, all bulbs do not light up. Pd across P is 3.0 V. Pd across all bulbs is zero.
Sort this out if you can.
