Final answer:
The simple subject in the sentence is 'Thinking'.
Step-by-step explanation:
The simple subject in the sentence 'Thinking about what you are grateful for is one of the keys to happiness' is 'Thinking'.
In this sentence, 'Thinking' is the subject because it is the noun that the verb 'is' is referring to. It is what the sentence is talking about.
For example, if we rewrite the sentence using a pronoun instead of the gerund 'Thinking', it would be: 'It is one of the keys to happiness.' Here, 'It' is the subject and it refers to the act of thinking.