What part of speech is the bolded phrase in the sentence below? Is it a gerund or a participle?

The bolded phrase "Thinking critically" in the sentence "Thinking critically is a skill." functions as a noun. This is because the entire phrase acts as the subject of the sentence. Specifically, "thinking" is a gerund in this context because it derives from a verb and functions as a noun.

So the correct answer is: **noun, gerund**.