Here are the correct answers for each sentence, with explanations:

23. **The teacher, _______ we studied math with last year, is very strict.**
- **b) whom**
- Explanation: "Whom" is used for people as the object of the preposition "with."

24. **The cake, _______ my mother baked, is delicious.**
- **a) which**
- Explanation: "Which" is used to add non-essential information about things (the cake) in this context.

25. **The girl, _______ I met at the party, is from Brazil.**
- **c) who**
- Explanation: "Who" is used for people as the subject of the clause.

26. **The house, _______ has a big garden, is very beautiful.**
- **a) which**
- Explanation: "Which" is used to provide extra information about the house, functioning as the subject of the clause.

27. **The cat, _______ is sleeping on the couch, is mine.**
- **a) which**
- Explanation: "Which" is used for things (the cat) to provide additional information.

28. **The book, _______ I borrowed from the library, is overdue.**
- **a) which**
- Explanation: "Which" is used to add non-essential information about the book.

29. **The dog, _______ barks all night, is annoying.**
- **a) which**
- Explanation: "Which" is used to describe the dog and its behavior.

30. **The restaurant, _______ serves the best pizza in town, is very popular.**
- **a) which**
- Explanation: "Which" introduces a clause providing additional information about the restaurant.

31. **The doctor, _______ I saw yesterday, prescribed me some medicine.**
- **b) whom**
- Explanation: "Whom" is used as the object of the verb "saw" for people.

32. **The store, _______ I usually shop at, has a sale this week.**
- **a) which**
- Explanation: "Which" is used to add information about the store.

33. **The movie, _______ we saw last night, was really good.**
- **a) which**
- Explanation: "Which" introduces extra information about the movie.

34. **The computer, _______ my brother uses, is very fast.**
- **d) whose**
- Explanation: "Whose" shows possession, indicating that the computer belongs to the brother.

35. **The team, _______ we played against, was very strong.**
- **b) whom**
- Explanation: "Whom" is used for people and functions as the object of the preposition "against."

36. **The city, _______ I grew up in, is very different now.**
- **a) which**
- Explanation: "Which" adds information about the city.

37. **The song, _______ is playing on the radio, is my favorite.**
- **a) which**
- Explanation: "Which" provides additional information about the song.

38. **The doctor, _______ patients are always happy with, is very kind.**
- **d) whose**
- Explanation: "Whose" indicates possession, referring to the patients belonging to the doctor.

39. **The plant, _______ needs a lot of water, is dying.**
- **a) which**
- Explanation: "Which" adds information about the plant.

40. **The child, _______ I’m taking care of, is very beautiful.**
- **b) whom**
- Explanation: "Whom" is used as the object of the preposition "of" referring to a person.

These answers help ensure correct usage of relative pronouns in different contexts.