The correct answer is: A. have proposed/should have begun
The sentence is in the present perfect tense, which is used to talk about things that have happened or been true up to the present time. The past participle of the verb “propose” is “proposed.” The infinitive of the verb “begin” is “begin.”
The sentence can be paraphrased as follows: “What I am saying is that we should have begun to think seriously about a thorough reform in the legal system.”
Option B is incorrect because the past perfect tense is used to talk about things that had happened or been true before a certain point in the past. In this case, there is no indication of a specific point in the past before which we should have begun to think seriously about a thorough reform in the legal system.
Option C is incorrect because the present progressive tense is used to talk about things that are happening now or that will happen in the near future. In this case, the speaker is talking about something that should have happened in the past, not something that is happening now or will happen in the near future.
Option D is incorrect because the past progressive tense is used to talk about things that were happening at a specific point in the past. In this case, there is no indication of a specific point in the past when we were beginning to think seriously about a thorough reform in the legal system.
Option E is incorrect because the simple past tense is used to talk about things that happened at a specific point in the past. In this case, the speaker is talking about something that should have happened in the past, not something that happened at a specific point in the past.