Which law of thermodynamics states that entropy in an isolated system always increases.

Entropy is a measure of the disorder or randomness of a system.

• The Zeroth Law deals with thermal equilibrium, stating that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
• The First Law deals with the conservation of energy, stating that energy cannot be created or destroyed, only transferred or changed from one form to another.
• The Second Law states that the total entropy of an isolated system can only increase over time, or remain constant in ideal cases where the system is in a steady state or undergoing a reversible process. It never decreases.
• The Third Law states that the entropy of a system approaches a constant value as its temperature approaches absolute zero.

The statement "entropy in an isolated system always increases" directly corresponds to the definition of the Second Law of Thermodynamics.

The correct answer is D.
$\boxed{\text{D. Second Law}}$