Explanation:

- Callous: An adjective used to describe someone who is emotionally hardened or indifferent to the suffering of others.

- Option 1, Brutal: This word means savagely violent, but it can also imply a lack of compassion, making it a fitting synonym for callous.

- Option 2, Problematic: Refers to something that causes difficulties or problems. It doesn’t relate to being emotionally hardened.

- Option 3, Large: Describes something of considerable size, unrelated to emotional disposition.

- Option 4, Humane: The opposite of callous; it means compassionate or kind.