Imperative Sentences in Direct Speech sometimes begin with Negative Commands like "Don't"
The reporting verbs like "asked/ask", "told/tell", "advised/ advise" etc. are changed into "said/say", etc.
"Not + to + V1" in indirect speech is changed to "Don't + V1" in direct speech.
Thus, going by the options is one of the best techniques to solve these questions.
For example:
My friend advised me not to go to Kolkata. (Indirect Speech)
My friend said to me, "Don't go to Kolkata"." (Direct Speech)
The following things in the sentence will change when changed from Indirect Speech to Direct Speech:
'Not to' will be changed to 'don't'.
'Next day' will changed to 'tomorrow'.
Inverted commas will be introduced.
Therefore, the correct answer is option (d): I warned Rita, "Don't be late tomorrow".