The phrase "Despite facing severe criticism" indicates a contrast. If the scientist continued her pursuit of truth even with criticism, she would need to be persistent and determined. "Relentless" means unceasingly intense or persistent, which accurately describes someone who continues their efforts in the face of opposition.
Hesitant: tentative, unsure, or slow in acting or speaking.
Casual: relaxed and unconcerned.
Indifferent: having no particular interest or sympathy; unconcerned.
"Relentless" best fits the context of perseverance.