Absolutely necessary; extremely important.
"it is essential to keep up-to-date records"
THAT IS A WIERD QUESTION IS IT MULTIPLE CHOICE
Your incorrect answers were 3 and 4.
For number 3, the correct answer is b. This is because not language is not necessarily oral (i.e. there are written languages).
For number 4, the correct answer is b. Although comparative linguistics is a branch of historical linguistics, comparative linguistics is specifically concerned with comparing the linguistics of one person/place from the next.
