Concept:Subject-verb agreement: a plural subject must take a plural verb.
Explanation:Check each sentence for subject-verb agreement.
In option A, the subject is "principles" (plural) but the verb "is" is singular. This is wrong. It should be "are".
Option B: "Reactions" (plural) matches "have been". Correct.
Option C: "The author" (singular) matches "is". Correct.
Option D: "Many lines" (plural) matches "are". Correct.
Thus, only sentence A has an error.
Answer:Option A: "The principles of science is difficult to understand" is the incorrect sentence.