Step 1: Understanding the Concept:
Stars and interstellar gas are made mainly of the lightest element, formed first after the Big Bang.
Step 2: Detailed Explanation:
Hydrogen makes up about 75 percent of the universe by mass, and helium makes up most of the rest. All heavier elements together are only a small fraction.
Sodium, magnesium and potassium are present only in trace amounts on cosmic scales. They are more common on Earth, but not in the universe as a whole.
Step 3: Final Answer:
Hydrogen is the most abundant element, option (A).
Final Answer:
Hydrogen is the most abundant element.
\[ \boxed{\text{(A) }\text{Hydrogen}} \]