Select the correct indirect form of the given sentence.

The professor said, "You must learn these words regularly." 

The professor said that I had to learn those words regularly.

The correct indirect form of the given sentence is:

The professor said that I should learn those words regularly.

The other options are incorrect because they do not follow these rules:

• The professor said that I must learn those words regularly. - This sentence uses the imperative mood, which is not correct in indirect speech.
• The professor said I should learn these words regularly. - This sentence changes the demonstrative pronoun "these" to "those," but it does not change the imperative mood to the subjunctive mood.
• The professor said that you had to learn these words regularly. - This sentence changes the present tense to the past tense and the second person pronoun "you" to the first person pronoun "I," but it does not change the imperative mood to the subjunctive mood.