Explanation:

Insects are a class of invertebrates in the animal kingdom. They are characterized by having six legs and two pairs of wings. Let's break down the characteristics of an insect and understand why option D is the correct answer.

1. Six Legs:
- One of the defining features of an insect is that it has six legs.
- These legs are attached to the thorax, the middle part of an insect's body.
- The legs enable insects to walk, run, jump, climb, and perform various other movements.

2. Two Wings:
- Most insects have two pairs of wings, which are attached to the thorax.
- The wings allow insects to fly and are an essential adaptation for many species.
- However, not all insects have wings. Some insects, like ants and fleas, have lost their wings through evolution.

Why option D is correct:
- Option D, "Both A and B," is the correct answer because it includes both the characteristics that define an insect: six legs (option A) and two wings (option B).
- Having both six legs and two wings is a characteristic unique to insects and helps differentiate them from other animals.

Other options:
- Option C, "Beak," is incorrect because insects do not have beaks. Beaks are typically found in birds and some other animal groups.
- Option A alone (six legs) or option B alone (two wings) would not be sufficient to classify an organism as an insect. Both characteristics must be present.

In summary:
- Insects are characterized by having six legs and two wings.
- These characteristics help insects perform various movements and enable them to fly.
- Option D is the correct answer as it includes both the defining features of an insect.