Correct Answer: The chemist said that they did not have stock of that medicine then.

Solution : The fourth option is the correct choice.

The steps of conversion are as follows:

• There would be no change in the reporting verb in the indirect speech.
• "We" (a first-person pronoun) is changed to "they" (a third-person pronoun).
• The present tense "do not" is changed to the past tense "did not".
• "This" is changed to "that".
• The connector "that" is used to introduce the reported clause.

Therefore, the correct answer is: "The chemist said that they did not have stock of that medicine then."