Explanation:

- "To gain ground" is an idiom that means to make progress or to advance in a situation or area. It's often used in contexts where someone or something is becoming more successful or getting closer to a goal.

- Option 1: To make progress. This is the correct meaning of the idiom "to gain ground."

- Option 2: To be happy. This meaning does not relate to the idiom. It's more about an emotional state rather than progress or advancement.

- Option 3: To grow crops. Although this involves land, it does not reflect the idiomatic meaning.

- Option 4: To buy land. Again, despite involving ground or land, this is a literal interpretation and not the idiomatic use.