Specialize means that you are good at something and are and expert at
Positive thinking is an attitude that pushes you to expect good and desired results. Power of positivity helps you in creating and transforming energy into reality. Positive mindset helps you to seek happiness, health and a happy ending regardless of the situation. Lots of successful people have recognized the positivity as the key to success.
An opinion is a certain statement or judgment which is not necessarily based on facts or knowledge. Words and phrases such as "I believe", "worse", or "than" indicate opinion. This will tell you if the nonfiction you are reading contains opinion. Therefore, the correct answer would be option D.
