The correct answer is: E. No error
The sentence is grammatically correct. The word “heart” is used as an adverb to modify the verb “learn,” meaning that the students were instructed to learn the passage carefully and thoroughly.
The other options are incorrect because they contain grammatical errors. Option A is incorrect because the word “since” is used as a subordinating conjunction, which means that it must be followed by a clause that begins with a verb in the past tense. Option B is incorrect because the word “were” is used as a plural verb, but the subject of the sentence is “the students,” which is singular. Option C is incorrect because the word “to” is used as an infinitive marker, but the verb “learn” is already in the infinitive form. Option D is incorrect because the word “passage” is a noun, but the verb “learn” requires a direct object.