The problem concerns the definition of an onto function (also known as a surjective function). Let's analyze the statements:

1. Every element in the co-domain must have an arrow pointing to it from some element of the domain.
   This is correct. An onto function ensures that every element of the co-domain is mapped to by at least one element of the domain. This is a defining property of surjective functions.

2. Every element in the domain must have an arrow pointing to it from some element of the co-domain.
   This statement is incorrect. This would describe an injective or one-to-one relationship in the reverse sense, which is not relevant for surjectivity.

Thus, the correct answer is A. I only.

Would you like further clarification? Here are some related questions for practice:

1. What is the definition of an injective function?
2. How does a bijective function differ from an onto function?
3. Can a function be both injective and surjective? If so, what is it called?
4. What are some real-life examples of surjective functions?
5. How can you verify if a function is surjective given its formula?

Tip: To check if a function is onto, ensure that every value in the co-domain is reachable by some input in the domain.