Out of the four alternatives given, choose the one which can be substituted for the words An animal that lives in groups.

Introduction / Context:
This one word substitution question focuses on a term used in zoology and general English to describe social behaviour. The phrase An animal that lives in groups refers to species that prefer company and often move or feed together. The vocabulary tested here also appears in descriptions of human personality, making it doubly useful for learners.

Given Data / Assumptions:

• Description: An animal that lives in groups.• Options: Hoard, Fastidious, Gullible, Gregarious.• The required word must describe a social, group living tendency.• We assume standard English dictionary meanings.

Concept / Approach:
The adjective gregarious is used for animals that live in flocks, herds, or groups, such as gregarious birds or gregarious insects. It is also used for people who enjoy the company of others and are sociable. Hoard is usually a noun or verb relating to storing things secretly. Fastidious means very careful and hard to please, especially in matters of cleanliness or detail. Gullible describes someone who is easily tricked. Only gregarious captures the idea of group living.

Step-by-Step Solution:
Step 1: Identify the essential feature in the description living in groups instead of alone.Step 2: Recall that gregarious animals are those that form herds or flocks, and gregarious people are those who enjoy social company.Step 3: Examine hoard, which refers to a hidden store of goods or the act of storing and does not relate to group life.Step 4: Consider fastidious, which deals with attention to detail and fussiness, not with social grouping.Step 5: Look at gullible, which describes being easily deceived and has no link to living in groups.Step 6: Conclude that gregarious is the only option that correctly names the characteristic of group living.

Verification / Alternative check:
Think of examples: Sheep are gregarious animals, and Some birds are highly gregarious. These sentences are used in science texts and nature documentaries. Trying to substitute hoard, fastidious, or gullible in place of gregarious would break the sense and sound absurd. The consistency of usage across biology and social descriptions confirms that gregarious is the standard term for animals that prefer group life.

Why Other Options Are Wrong:
Hoard, whether as a noun or verb, concerns accumulation and storage, often in secret, and does not describe social structure. Fastidious refers to being very particular or demanding about cleanliness or correctness, again not involving social behaviour. Gullible focuses on mental tendency to believe things easily and is used for individuals, not patterns of group living. None of these words carry the meaning of living in groups.

Common Pitfalls:
Because the question describes animals, some candidates may choose hoard, associating it with animals that store food. However, the wording clearly emphasises living in groups, which indicates social behaviour rather than storage behaviour. A helpful memory trick is to connect gregarious with group and gather, as all suggest being together with others.

Final Answer:
The correct one word substitute is Gregarious.