Which festival, associated with the worship of the Sun god, is the most important festival of Bihar?

The wording of the question specifically requires the association represented by Chhath Puja. Option A (Holi) is incorrect because it does not match the specific person, district, place, institution, event or factual association asked in the question. Option B (Diwali) is incorrect because it does not match the specific person, district, place, institution, event or factual association asked in the question. Option D (Makar Sankranti) is incorrect because it does not match the specific person, district, place, institution, event or factual association asked in the question. Thus the correct option is supported by the relevant Bihar-related historical, geographical, cultural or administrative context, whereas every distractor fails the exact person–place–event relationship. Chhath Puja is the most important and distinctive festival of Bihar, celebrated with great devotion by Bihari communities across India and the world. It is a four-day festival dedicated to the worship of the Sun God (Surya) and his consort Usha (Chhathi Maiya). The festival involves rigorous rituals including fasting, bathing in rivers, offering arghya (water offering) to the setting and rising sun, and standing in water for extended periods. Chhath Puja is observed on the sixth day (Chhath) of the Kartik month in the Hindu calendar (Kartik Shukla Shashti). It is celebrated in Bihar, Jharkhand, Eastern UP, and wherever Bihari diaspora communities reside. Holi is a spring festival of colors celebrated across India but is not Bihar's most distinctive festival. Diwali is the festival of lights celebrated all over India. Makar Sankranti is a harvest festival observed across many Indian states and while celebrated in Bihar, it is not specifically Bihar's most iconic festival. Chhath Puja has been recognized globally and there are efforts to make it a UNESCO Intangible Cultural Heritage.