B₁, B₂ and B₃ are the three identical bulbs connected to a battery of steady emf with key K closed. What happens to the brightness of the bulbs B₁ and B₂ when the key is opened?

Brightness of the bulb B₁ decreases and B₂ increases

When key K is opened, bulb B₂ will not draw any current from the source. So, that terminal voltage of source increases. Hence, power consumed by bulb increases. So, light of the bulb becomes more. The brightness of bulb B₁ decreases.