Our mayor is a really conscientious ruler because he provided protective masks for [the] inhabitants.
A is needed before "really consicentious ruler", because "ruler" is a singular count noun, and there is no context that licenses dropping the article.
An article is not possible before "protective masks", because that is plural and does not denote specific masks. Edit: Jason points out that there could be an article there if it referred to particular masks, such that the speaker expects the hearer to know which ones are meant. I missed that reading.
The is possible before "inhabitants", but changes the meaning. With "the", it is definite, and implies "all the inhabitants" (or, rather "all the inhabitants relevant to the established context"). Without, it is indefinite, and implies only some inhabitants - maybe all, but not necessarily.