The correct answer to the question is option 'b) Ekarat'. The term 'Ekarat' was used by the Aitariya Brahmana to describe a monarch who had made conquests in all the four directions. Let's understand this answer in detail.

Explanation:
The Aitariya Brahmana is a Vedic text associated with the Rigveda. It contains various rituals, sacrifices, and philosophical discussions. In one of its passages, it mentions the term 'Ekarat' to describe a powerful monarch who had successfully conquered territories in all the four directions.

1. Meaning of 'Ekarat':
The term 'Ekarat' can be broken down into two parts - 'Eka' meaning 'one' and 'Rat' meaning 'chariot'. Therefore, 'Ekarat' can be translated as 'one-chariot' or 'single-chariot'. This term symbolizes the prowess and power of a monarch who had achieved victory in all directions with just one chariot.

2. Significance of Conquests in All Four Directions:
Conquering territories in all four directions was considered a significant achievement for a monarch. It demonstrated their military might, political influence, and ability to maintain control over a vast empire. Such conquests often expanded the kingdom's boundaries, increased its resources, and enhanced its prestige.

3. Aitariya Brahmana's Reference:
The Aitariya Brahmana, through the term 'Ekarat', acknowledges and praises the monarch who had achieved conquests in all four directions. This reference highlights the importance given to military conquests and territorial expansion in ancient Indian society.

4. Other Terms for Conquering Monarch:
While 'Ekarat' is the correct answer in the context of the Aitariya Brahmana, it is worth noting that other terms were also used to describe such monarchs in different ancient Indian texts. For example, the term 'Samrat' was used to refer to an emperor or sovereign ruler who held supreme power and authority over a vast empire.

In conclusion, the Aitariya Brahmana used the term 'Ekarat' to describe a monarch who had made conquests in all four directions. This term symbolizes the power and success of such a ruler in expanding their kingdom and establishing dominance over a wide expanse of territories.