In the circuit diagram, all the bulbs are identical. Which bulb will be the brightest?

As all bulbs are identical and the power (related to brightness) is given as P = I2 R, so the resistor with the most current will be brightest. As bulb C has the same current as that through the battery while the branches with bulbs A, B and D are in parallel and have the total current split between the branches, the current through bulb C is greatest.