**Explanation:**

In this situation, the speaker is reflecting on the performance of the rest of the class and comparing it to their own performance. They are realizing that they didn't do as well as they expected and are expressing regret or acknowledging a missed opportunity. The correct answer is "should have studied" because it indicates that the speaker should have studied more in the past in order to achieve a better result.

**Modal Verb "Should":**

- The modal verb "should" is used to express advice, obligation, or expectation.
- In this case, it is used to express an obligation or expectation that the speaker should have studied more.

**Past Perfect Tense "Have Studied":**

- The past perfect tense is formed by using the auxiliary verb "have" in the past tense (had) followed by the past participle of the main verb (studied).
- It is used to indicate an action that was completed before another past action or point in time.
- In this case, "have studied" indicates that the studying should have been completed before the time of reflection on the class's performance.

**Contrasting with the Rest of the Class:**

- The speaker mentions that the rest of the class did pretty well, implying that they achieved satisfactory results.
- By using the phrase "did pretty well," the speaker compares their own performance to that of the rest of the class and realizes that they didn't perform as well.
- This contrast highlights the missed opportunity for the speaker to have studied more and potentially achieved better results.

**Expressing Regret:**

- The speaker's use of the word "harder" suggests that they recognize their lack of effort or dedication in studying.
- By using the past perfect tense "should have studied," the speaker expresses regret or acknowledges their failure to meet the expectation of studying more in the past.
- This regret is based on the realization that studying more would have likely led to a better performance.

In conclusion, the correct answer "should have studied" reflects the speaker's realization that they didn't perform as well as the rest of the class and expresses regret or acknowledgment of their failure to study more in the past.