The correct answer is (a).
The Eighth Mandala of the Rigveda is dedicated to the god Soma. Soma is a deity of the Vedic pantheon, associated with the moon, intoxicating drink, and fertility. He is often invoked in hymns to other deities, and is also the subject of many hymns in his own right.
The Eighth Mandala is the longest of the ten mandalas of the Rigveda, and contains 124 hymns. The hymns in this mandala are mostly addressed to Soma, and are characterized by their use of complex imagery and symbolism.
The Eighth Mandala is an important source of information about the Vedic religion and culture. It provides insights into the Vedic view of the world, and the role of Soma in Vedic ritual and mythology.
The other options are incorrect.
(b) The Ninth Mandala is dedicated to the god Indra.
(c) The Tenth Mandala is a collection of miscellaneous hymns.
(d) The Sixth Mandala is dedicated to the god Agni.