> she/he (not sure)
English is really strange around gender, and idiomatic styles have changed over time. It has always been a valid style in English to use "they" to refer to a single person of unknown gender [1]. This feels natural to English speakers when the subject is unknown and could be one of many potential people, as in:
When the first guest arrives at the party, give them a balloon.
Here, "them" only refers to a single person, but it's correct.
For the past hundred years or so, it was also common to use "he" to refer to an unknown hypothetical person. You see a lot of textbooks that do this. In theory, this wasn't supposed to come across as being exclusionary, but obviously it is. It's an explicitly gendered pronoun.
The feminist movement rightly drew attention to this problem, and writers experimented with a variety of approaches. Sometimes, at the beginning of a book you'll see an explicit "apology" saying something like "we use 'he' but don't mean it to only apply to males". Some simply switched to using "she" for everything. Others switch between "he" and "she" throughout, randomly or use the longer "he or she".
But, lately, it seems like the style is settling down towards simply using the already established singular they for all of these cases. To a native speaker, it feels a little weird at first, but you quickly get used to it. It's less jarring than "he" or "she" for most readers now.
It has a lot going for it:
* It has more established history than any other form.
* It doesn't force the author to pick an arbitrary meaningless gender.
* It's shorter than the awkward "he or she" (which also still forces you to decide which gender to put first).
* It doesn't exclude people who prefer neither "he" nor "she" as their pronoun.
So, if you're trying to figure out what pronoun to use when you don't know a specific one that is correct, default to "they".