A second language ___________________________everybody.

The phrase "A second language should be taught to everybody" conveys the idea that it is advisable or recommended for everyone to receive instruction in a second language. The term "should" indicates a strong suggestion or recommendation, which aligns perfectly with the context of teaching and education.

In this case, the focus is on the importance of making second language education accessible to all individuals, highlighting the belief that everyone would benefit from such knowledge. This implies a sense of duty or responsibility in educating people about second languages.

Other options, while they may seem plausible at first glance, do not maintain the same emphasis on the recommendation to teach a second language to everyone. For instance, "must be learned by" suggests an obligation placed on individuals rather than highlighting the necessity of education as a process. Similarly, "could be understood by" implies potential rather than addressing the act of teaching. Lastly, "would be taught to" presents a conditional scenario without the direct recommendation that "should be taught to" provides.