Which festival is popularly known as the festival of light?

The correct answer is Diwali.

To understand why, let's look at the cultural significance of the options provided:
1. Bihu: A set of three important cultural festivals in the Indian state of Assam, primarily celebrating the harvest and agriculture.
2. Navratri: A nine-night Hindu festival dedicated to the worship of the Goddess Durga, symbolizing the triumph of good over evil.
3. Diwali: Also known as Deepavali, this festival literally translates to a "row of lights." It is celebrated by lighting oil lamps (diyas), candles, and fireworks to symbolize the inner light protecting us from spiritual darkness, marking the victory of light over darkness and good over evil.
4. Holi: Popularly known as the "Festival of Colors," celebrating the arrival of spring and the victory of good over evil.

Therefore, Diwali is the festival popularly known as the festival of light.