Which mapping idea correctly identifies a function?

Correct answer: Only one arrow leaves each domain element

Direct answer: Option A, only one arrow leaves each domain element. A function from a domain to a codomain assigns exactly one output to every input. In a mapping diagram, this means every element of the domain must have one and only one outgoing arrow. The outputs may be the same for different inputs, and some codomain elements may receive no arrow. Option A states the essential rule. Option B is not allowed because one input would have two different outputs, so the assignment would not be single-valued. Option C is not allowed because an input with no arrow has no assigned output, so the rule is incomplete. Option D describes a surjective or onto condition, not the basic definition of every function; a function need not cover the whole codomain. For example, two domain elements may both point to one codomain element and the function can still be valid. Memory cue: one input, exactly one output; many inputs may share an output.