what does london mean when he states his writing was in truth a protest against the humanizing of animals

He means that he did not falsify nature, but introduced animals that acted on instinct, just like in real life.

Explanation:

London says that the animals in their stories are not humanized, they do not have the human capacity to reason, to think about the consequences, advantages and disadvantages of their actions. On the contrary, the animals in his stories, act completely on impulse, without the need for reasoning and caution. This is a way of being true to nature and not putting a human responsibility on animals, as some works approach. He states that when he writes in this way he criticizes the humanization of animals, that is, he criticizes the unrealistic and dishonest representation about humanity.

it translates into:

read the letter of your pen pal. write and answer 3 questions.