Answer:
The subject-verb agreement is wrong. The correct sentence would be:
- Excitement between the two groups has risen after the election.
Step-by-step explanation:
If we ask the verb a simple question, we can find the subject: "What has risen after the election? Excitement between the two groups." Even though the answer is long, the simple subject itself is comprised of just one singular word: "excitement". This is the most important word, the one to which the verb refers. We can even remove the words that come after it, and the effect or meaning will be the same: "Excitement has risen after the election."
The problem with "Excitement between the two groups have risen after the election" is that "have" is agreeing with "groups". However, as seen above, "groups" is not the most important word in the subject. Therefore, the verb should not agree with it.