The doctors advised him not to undertake so arduous a trip. (Change into direct speech)

1. Reporting Verb and Reporting Speech: In direct speech, the reporting verb ("said") is used to introduce the speaker's original words, and the reported speech is enclosed within quotation marks.

2. Pronoun Changes: In changing from indirect to direct speech, the pronouns may need to be adjusted. In this case, "not to undertake" in the indirect speech becomes "Do not undertake" in the direct speech.

3. Punctuation: The reported speech is enclosed within quotation marks, and a comma is used to introduce the reported speech.

Now, let's briefly analyze the other options:

- ক The doctors advised him. "Not to undertake so arduous a trip." - This option correctly uses the reporting verb "advised" but does not accurately reflect the change in pronoun and wording.

- খ The doctors said to him. "Undertake no arduous trip." - This option changes the meaning and wording of the original sentence. It doesn't accurately represent the advice given by the doctors in the indirect speech.

- গ The doctors advised to him, "Do not undertake so arduous a trip." - This option is grammatically incorrect. The correct structure is "advised him" instead of "advised to him."

In summary, option ঘ adheres to the rules of changing indirect speech to direct speech by using the correct reporting verb, adjusting pronouns, and accurately representing the original advice given by the doctors.