Answer:
The axiom is:
If two points lie in a plane, the line containing those points lies in the same plane.
Explanation:
Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions (theorems) from these.
Euclid axioms seemed to be so true or obvious that any theorem proved from them was deemed true in an absolute.
The statement that is an axiom of Euclidean geometry is:
If two points lie in a plane, the line containing those points lies in the same plane.