Choose the grammatically correct sentence-

The correct answer is Option (3) → The important lessons are as follows

As follows is a fixed expression used to introduce a list or details that come next. It is always "as follows", whether the subject is singular or plural.
Example: The rules of the game are as follows.

Why the Other Options are Incorrect?

Option 1: The important lessons are following — Incorrect. "Are following" means the lessons are coming after someone or obeying something; it does not introduce a list.

Option 2: The important lessons are as following — Incorrect. "As following" is not a form of the fixed expression.

Option 4: The important lessons are as follow — Incorrect. The expression does not change with a plural subject; "as follow" is wrong.