Which of the following statements about natural numbers is/are true? I. Closed under addition & multiplication. II. Closed under subtraction. III. Every non-empty subset has a least element.

Statement I says: "The set of natural numbers is closed under addition and multiplication."
Natural numbers (usually $\{1, 2, 3, \dots\}$ or $\{0, 1, 2, 3, \dots\}$) are closed under addition because the sum of any two natural numbers is always a natural number (e.g., $2+3=5$). They are also closed under multiplication because the product of any two natural numbers is always a natural number (e.g., $2 \times 3=6$).
Therefore, statement I is correct.

Statement II says: "The set of natural numbers is closed under subtraction."
Closure under subtraction means that if you subtract any natural number from another, the result is always a natural number. This is not true. For example, 2 and 5 are natural numbers, but $2-5 = -3$, which is not a natural number.
Therefore, statement II is incorrect.

Statement III says: "Every non-empty subset of natural numbers has a least element."
This is known as the Well-Ordering Principle for natural numbers. Any non-empty collection of natural numbers will always contain a smallest element. For example, in the set $\{10, 5, 15\}$, the least element is 5.
Therefore, statement III is correct.

Statements I and III are correct, while statement II is incorrect.
Comparing this with the given options:
(A) Only I
(B) I and II
(C) I and III
(D) II and III
The option that includes only I and III is (C).

The final answer is $\boxed{\text{C}}$