How do you solve the problem (Java, inheritance)?

You only have to understand polymorphy and inheritance for this task.

Let’s say there are these two classes that are in a parent-child relationship:

class Animal {
  public void run() {
  }
}

class Elephant extends Animal {
  public void toot() {
  }
}

The subclass elephant inherits all methods and properties that can be accessed from the outside Animal.

Elephant dumbo = new Elephant();
dumbo.run();

In this sense it is also legitimate to say that an elephant corresponds to an animal.

Animal dumbo = new Elephant();
dumbo.run();

That doesn’t work otherwise. Not every animal is also an elephant. Only the elephant knows how to comfort.

Elephant dumbo = new Animal(); // error
dumb.toot();

For the second part of the task, you can add the Wikipedia article Class diagrams use. Relevant to you is the Generalization.

That’s right.

No, O is a basic class T.

To describe it again with simpler names:

Animal > Elephant > African (Savanna) Elephant

An African elephant is both an elephant and an animal. An elephant is an animal. All these assignments therefore work:

Animal animal = new AfricanElephant();
Elephant animal2 = new AfricanElephant();
Animal animal3 = new Elephant();

Class T is synonymous with the African elephant. Where M and O in this inheritance chain, you can determine the illegal rules.