The opening hymn of the Rigveda, and the first hymns of most of its family books, are addressed to the god:

The very first hymn of the Rigveda is addressed to Agni, the god of fire, and the first hymns of most of the family books (Mandalas 2 to 7) are also addressed to him.

About Agni:

• "Agni" means fire; the word is related to the Latin "ignis".
• He is the god of the sacrificial fire, and the second most important god of the Rigveda, with about 200 hymns.
• He was the messenger between humans and gods: offerings put into the sacred fire were believed to be carried by Agni to the gods.
• He was present in every home as the household fire, and was called a guest in every house.
• He was born from two sticks rubbed together (the "arani").
• He has three forms: fire on earth, lightning in the atmosphere, and the sun in the sky.
• He destroys darkness and evil spirits, and purifies.
• He was believed to cremate the dead and carry them to the world of the ancestors.

Why Agni comes first:

• Since every sacrifice began with lighting the fire, Agni was invoked first.
• The order of hymns in the family books usually places Agni first, then Indra, then other gods.

The first verse:

The opening verse of the Rigveda praises Agni as the priest of the sacrifice, the divine minister and the giver of wealth.

Agni in Vedic life:

• Fire altars were central to Vedic religion.
• Three sacred fires were kept by householders in the "Shrauta" rituals: Garhapatya (household fire), Ahavaniya (offering fire) and Dakshinagni (southern fire).
• The story of Videgha Mathava in the Shatapatha Brahmana shows Agni moving eastwards.

Agni in later times:

He remained the god of fire, and fire continued to be the witness in Hindu marriages, as the couple walk around it (the "saptapadi" and "pheras").

Why not the other options?

• ✗Indra has the largest number of hymns.
• ✗Soma has the ninth mandala devoted to him.
• ✗Varuna is the guardian of cosmic order.