Correct Answer: The old man said that he had been walking in his garden at six o'clock.

Solution : The correct choice is the second option ​​​​.

The steps of conversion are as follows:

• The reporting verb "said" remains the same because the sentence has an assertive nature.
• The first-person pronouns "I" and "me" are changed to the third-person pronouns "he" and "his", respectively.
• The past continuous tense (was walking) is changed to the past perfect continuous tense (had been walking).
• The connector "that" is used to introduce the reported clause.

Therefore, the correct sentence is: " The old man said that he had been walking in his garden at six o'clock."