Verification of truth – "An animal always has ___." Identify the feature present in all animals.

Introduction / Context:
This question asks for a structural attribute that is universal across the animal kingdom. Some options name organs that many (but not all) animals possess.

Given Data / Assumptions:

• Lungs: absent in many animals (fish, numerous invertebrates).
• Brain: lacking in simple animals (e.g., sponges; nerve nets in cnidarians instead of centralized brains).
• Heart: absent in many invertebrates and primitive forms.
• "Skin": generalized external covering (integument) present across animals, though its form varies (epidermis, cuticle, exoskeleton with epidermal layer beneath).

Concept / Approach:
In biology, the integumentary system—an outer covering that protects and interfaces with the environment—is universal to animals, though its structure differs (mammalian skin, reptilian scales, arthropod cuticle over epidermis, etc.). Hence “skin” as a generalized outer covering is the only broadly correct universal among the given choices.

Step-by-Step Solution:

Verification / Alternative check:
Even organisms with shells or exoskeletons have an epidermal tissue layer—an integument that qualifies as “skin” in a broad, exam-style sense.

Why Other Options Are Wrong:
Lungs/brain/heart exclude wide swaths of the animal kingdom; picking them would make the statement false for many taxa.

Common Pitfalls:
Interpreting “skin” narrowly as mammalian skin; exam conventions use it to mean “outer body covering.”

Final Answer:
Skin