Elle, 

the original script is in French - so ‘croire’ is “correct”, as it is the word chosen by the original speaker/writer (Aurélie). It also fits well with this being an ‘opinion’ - but the distinction between using croire or penser is not absolute and they are often interchangeable.

The question then is whether in English we could say ‘ think ‘ in this situation. It doesn’t have to be the only choice - it just has to be a reasonable choice to be an acceptable translation. Translations are contextual and subjective.

Some would say ‘think’ in the context, others may say ‘ believe ‘ - either is fine. (For what it’s worth, I would mostly use ‘think’, so am in agreement with Aurélie’s translation as well).

‘Croire’ and ‘penser’ in French, and ‘think’ and ‘believe’ in English, have slightly different but overlapping meanings and usage.

 Penser que/Croire que = To think that/To believe that in French

 https://www.wordreference.com/fren/croire

 https://www.wordreference.com/fren/penser

I think in the context of the exercise  "Je crois vraiment que..." and "Je pense vraiment que..." are synonyms and both mean "I really believe"/"I really think". So both would work.

I found a whole youtube video on the differences between "je pense", "je crois" and "je trouve":

https://www.youtube.com/watch?v=OGN8-CU-am0

I think the context of the exercise matches the example with giving an opinion on the importance of wearing masks where "je pense" and "je crois" are considered synonyms.

Curious what others have to say about the difference between "je pense" and "je crois" in this context.

EDIT:

There is the following comment for the youtube video which contradicts a bit what is said in the video and the guy should know because he wrote a whole thesis on this topic.

"Croire, penser, savoir, supposer, trouver, sembler, paraître,... sont appelés "verbes cognitifs*. 

En effet, ces verbes que j'analyse dans dans ma thèse de doctorat traduisent des processus cognitifs d'accès à l'information subsumée dans le propos du sujet parlant. C'est cette fonction pragmatique qui a justifié ma l'appellation de" verbes médiatiques" dans ma thèse. 

La différence fondamentale entre penser et croire réside sur le double semantisme de croire. En effet, avec croire, le sujet parlant exprime un savoir par inférence et basé sur des éléments qui, bien que non énoncés ne demeurent cependant pas inexistants. 

Avec croire, une certaine croyance non rationnelle s'ajoute ou se supperpose au savoir par inférence inhérent à ces verbes."

📖L'EXPRESSION DE L'INFÉRENCE EN FRANÇAIS

Université Paris Nanterre La Défense et Université Potsdam (Berlin), 2019

"https://publishup.uni-potsdam.de/opus4-ubp/frontdoor/deliver/index/docId/46961/file/messi_diss.pdf