In Which Sentence Is The Use of Pronouns Correct?

1. Golden dart frogs choose to live in groups because it is a very social animal.

frogs (plural) it is (singular) - incorrect

2. A certain kind of snake is resistant to the frog's venom, and therefore it is his only predator.

a certain kind (singular) it is (singular) - correct

3. The golden dart frog is poisonous, even though their appearance seems harmless.

frog is (singular) their appearance (plural) - incorrect

4. A scientist must wear gloves to protect his skin from touching the skin of the frogs.

the skin (singular) the frogs (plural) - incorrect

I doubt that this is the point of the question. There was a convention (which many say is obsolete) that when the sex is unspecified it is assumed to be male.

Or further, that male identifiers (?) are deemed to include the female.

e.g. man, mankind, he, him, his etc.
"To each his own"

Include, or embrace (cue titters). Allegedly, Winston Churchill was speaking in Parliament in the 1920s and Lady Astor criticised him for using “Man” to mean “the human race”. He responded, “Grammarians will attest that Man embraces woman, unless otherwise stated in the text.” (It isn't in any Act Of Parliament, that's an urban myth).

in English the use of "he" to refer to a person of unknown or unspecified gender was prescribed by manuals of style and school textbooks from the early 18th century until around the 1960s. It is merely a convention, and one that can be adhered to, or not, as the writer or speaker desires, e.g. by use of the singular "they", but in many other languages it's built-in, especially ones where nouns are gendered, such as the Romance languages. In French a crowd of:

1000 men:- is male (of course)
1000 women: is-female (of course)
1000 people gender unspecified: is male
999 women and 1 man: is male

It gets quite complicated in German, where you have to balance gender-neutrality and wordiness.