Explanation:

- Pets are good guides for lonely people: This doesn't fit well. A "guide" typically refers to someone who shows the way or directs, like a guide dog for the visually impaired.

- Pets are good nurses for lonely people: This isn't accurate. "Nurses" imply caregiving in a medical sense, which pets are not equipped to do.

- Pets are good substitutes for lonely people: Substitutes might imply pets replace human interaction, which is partially true. However, this isn't the most fitting or common term used here.

- Pets are good companions for lonely people: This is the perfect fit. "Companions" suggest friendship and emotional support, which pets often provide.