Which of the following is an appropriate thesis statement using the “limited subject + precise opinion” formula?

men are people.

pets are cool.

group skydiving is surprisingly safe.

libraries are full of books and other reading material

Group skydiving is surprisingly safe. This is the correct answer.

The " limited subject " is group skydiving. The thesis has taken a specific part from a general topic: dangerous  group sports. Then, what will be said and developed about this subject? The fact that is surprisingly safe. This is a precise opinion, which is expressed through the adverb surprisingly. If the thesis included only safe, the opinion would not be  precise. The adverb tells the writer's opinion about the safety of the sport. The writer seems to be surprised about the safety of a sport which sounds too dangerous and risky.

These options are not right:

Men are people. ( The subject men is general and there are not opinions).

Pets are cool ( The subject pet is not limited: Which pets? and then, cool is not a precise opinion. We cannot see clearly how cool the writer thinks  the pets are).

Libraries are full of books and other reading material ( This is a description. Which libraries? There is not any opinion).

recent brain research has shown some surprising benefits as a result of formal musical training. learning to play a musical instrument appears to develop parts of the brain that can even influence academic results. the studies indicate that the earlier a child begins training, the more benefits there are. one such study showed that children who began musical studies before the age of seven had a more developed brain in the region where language production and auditory processing take place. scientists believe there is mounting evidence to show that early musical training can improve brain function.

explanation: