Answer:
The answer is a predicate nominative.
Explanation:
Let`s split this sentence into its basic parts: subject (the dream of space travel), predicate (became a reality).
Now let`s analyze predicate closely:
it consists of a verb (linking verb) and a noun that modifies and explains it.
Since it`s a noun, the correct answer is a predicative nominative.
Of course, if the adjective was next to the predicate, the answer would be a predicate adjective.
