Out of the following bulbs rated $40\, W , 220\, V$ $60\, W , 220 \,V$ and $100\, W , 220\, V$ which one will glow the brightest when connected in series to a supply of $220\, V$ $?$

The resistance of a bulb is given by the expression, $R =\frac{ V ^{2}}{ P } .$ since all have the same voltage rating, therefore, the $40 W$ bulbs has maximum resistance. When these three bulbs are connected in series, same current passes through each. As heat produced is directly proportional to the resistance when the current is constant, therefore, the $40 W$ bulb will produced maximum heat and hence will be the brightest.