Seeing as unquestionably starts with a vowel, U, you should use 'an' before it.
She was unquestionably the victor.
Lead is unquestionably heavier than an equal volume of feathers.
5
Unquestionably you can
This word is an adverb.
Unquestionably
Unquestionably
Without doubt or question; unquestionably.
Unquestionably NO!
Unquestionably Mrs. Jarley had an inventive genius.
If you mean by word "unquestionably".
unquestionably!