Select the most appropriate synonym of the underlined word in the following sentence.
He behaved ruthlessly with his junior, to say the least.

The correct option is Inhumanly.

Explanation:
The word ruthlessly means behaving in a harsh, cruel, or pitiless manner without showing any mercy or compassion.

Let's analyze the given options:

1. Inhumanly: Cruelly, lacking human feeling, sympathy, or compassion. This is the closest in meaning to 'ruthlessly'.
2. Unexpectedly: In an unforeseen manner or by surprise.
3. Weirdly: In a strange, bizarre, or unusual manner.
4. Politely: In a courteous, well-mannered, and respectful way.

Therefore, the most appropriate synonym of the underlined word ruthlessly is Inhumanly.