In a grammar, if the production S->λ where S is a non-terminal is existed, such production is called ?

In formal grammar, a production rule of the form $A \to \lambda$ (or $A \to \epsilon$), where $A$ is a non-terminal symbol and $\lambda$ (or $\epsilon$) represents the empty string, is known as a null production or epsilon production. This type of production indicates that the non-terminal $A$ can derive the empty string.

Let's look at the options:

• A. Null Production: This matches the definition of a production where a non-terminal derives the empty string.
• B. Unit Production: A unit production is of the form $A \to B$, where both $A$ and $B$ are non-terminals.
• C. Secondary Production: This is not a standard term in formal language theory.
• D. Sigma Production: This is not a standard term in formal language theory. Sigma ($\Sigma$) typically denotes the set of terminal symbols.

Therefore, the production $S \to \lambda$ is a null production.

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