Consider the following statements about elephants:

A. Elephants live in herds.

B. A herd may have 20 to 25 female elephants and their young ones.

C. Male elephants live in the herd till they are 9–10 years.

D. The oldest female takes all the decisions of the herd.

The correct statements are

To determine the correct statements about elephant herds, let's analyze each of the given statements based on the behavioral biology of elephants:

Statement A: Elephants live in herds.
This statement is correct. Elephants are highly social animals that live in organized social groups called herds.

Statement B: A herd may have 20 to 25 female elephants and their young ones.
This statement is incorrect. Typically, an elephant herd is smaller, usually consisting of about 10 to 12 female elephants and their young offspring.

Statement C: Male elephants live in the herd till they are 9–10 years.
This statement is incorrect. Male elephants generally stay in the herd until they reach adolescence, around 14 to 15 years of age, after which they leave the herd to live independently or join loose bachelor groups.

Statement D: The oldest female takes all the decisions of the herd.
This statement is correct. Elephant herds follow a matriarchal structure where the oldest and most experienced female (the matriarch) leads the herd and makes decisions regarding travel, finding food, and safety.

Therefore, only statements A and D are correct, which corresponds to the option "A and D".