The correct answers are What were once considered bizarre science fiction tropes have now become everyday realities. and What was once considered a bizarre science fiction trope has now become an everyday reality.

When a nominal relative clause starting with "what" acts as the subject of a sentence, the verb agreement depends on the meaning of "what." If "what" refers to a plural concept (the things that), the verbs inside and outside the clause should be plural. If it refers to a singular concept (the thing that), the verbs should be singular. Mixing singular and plural verbs across the clause and main sentence creates grammatical disagreement.