Correct Answer: You said that you would bake a walnut cake.

Solution : The fourth option is the correct answer.

Indirect speech, also known as reported speech or indirect discourse, is a way of reporting what someone else has said without quoting their exact words.

The steps of conversion are as follows:

• There would be no change in the reporting verb.
• The first-person pronoun "I" is changed to the second-person pronoun "you".
• The modal verb "will" is changed to its past form, "would".
• The connector "that" is used to introduce the reported clause.

Therefore, the correct answer is: You said that you would bake a walnut cake.