It became ______ that I was not welcome
There were four words: clear, clearly, certainly, certainty.
I thought that it was "clearly"; I don't know if it was wrong.
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Become is usually followed by either a noun phrase (eg "He became a teacher") or an adjective phrase ("I became angry". "It became clear").
There may be cases where it can be followed by an adverbial complement, but I can't think of any.
So it has to be clear or certain.
Certain is grammatically possible, but does not fit the meaning.
"It became clear that ... " refers to somebody's growing understanding of a situation.
"It became certain that ... " refers to the increasing likelihood or definiteness of a situation; for example "As the results came in it became certain that they were going to win".