1. Since he is a human, he is in charge.

2. He always drinks, and is horrible to the animals on the farm.

3. After Jones is pushed away from the Animal Farm, a new leader, or monster, is created.

4. As one of the pigs on the farm, Napoleon was a fierce looking boar, and leader.

5. Even when Jones is in charge of the farm, Napoleon gets his way.

6. No leader should go without power.

The prepositional phrase is a series of words, so it can't just be an adjective or an adverb.

1. In charge

2.to the animals, on the farm

3. Of the pigs, on the farm

5. In charge, of the farm

6. without power

A prepositional phrase can be used as an adjective to modify the subject or it can be used as an adverb to modify the predicate.

1. , he is in charge.
This is a clause. It has a subject and verb. Phrases don't. It modifies charge answer the question why.

2. He always drinks, and is horrible to the animals on the farm. Ask what to the animals on the farm? Horrible so that phrase is adverbial. Adjectives add meaning to nouns. Adverbs add meaning to everything else except nouns. Horrible is a predicate adjective describing he.

3. , a new leader, or monster, is created. What from the Animal Farm?
Pushed away, which is a verb with an adverb so the pp has to be adverbial.

See what you come up with now.

4. As one of the pigs on the farm, Napoleon was a fierce looking boar, and leader.

5. Even when Jones is in charge of the farm, Napoleon gets his way.

6. No leader should go without power.