In the following question, a sentence is given in direct speech: Ram said to Rohan, "Do not run so fast." Out of the four alternatives, select the option that best expresses this sentence in indirect speech.

Introduction / Context:
Reported speech or direct indirect speech questions test whether you can change a quoted sentence into a reported form while preserving the original meaning and applying correct grammar. Here the original sentence is an imperative or command in the negative form. You must choose the reported version that correctly converts the imperative structure and fits naturally with the reporting verb said to.

Given Data / Assumptions:

• Direct speech: Ram said to Rohan, "Do not run so fast."
• Reporting verb: said to, which often becomes told in indirect speech when followed by an object.
• The quoted part is a negative imperative (a request or order not to do something).
• We need an indirect speech sentence that is grammatically correct and matches the tone of the original.

Concept / Approach:
When we convert negative imperatives in direct speech into indirect speech, we normally use a structure like told or asked someone not to plus base verb. The reporting verb said to is usually changed to told when it introduces an instruction or command. We also remove quotation marks and the auxiliary do, and instead introduce the infinitive not to run. We should not introduce why clauses or question forms because the original sentence is not a question. The most neutral and grammatically correct option here is Ram told Rohan not to run so fast.

Step-by-Step Solution:
Step 1: Identify the type of sentence inside the quotes. "Do not run so fast" is a negative command or instruction. Step 2: Replace said to with an appropriate reporting verb. In indirect speech for commands, said to is normally changed to told. So we begin with Ram told Rohan. Step 3: Convert the imperative. For negative imperatives, use not to plus the base form of the verb. So "Do not run so fast" becomes not to run so fast in indirect speech. Step 4: Combine the parts: Ram told Rohan not to run so fast. This sentence is grammatically correct, uses the correct reporting verb and keeps the same meaning. Step 5: Compare this with the given options and see that it matches option D exactly.

Verification / Alternative check:
We can quickly test each option against the rules for reporting imperatives. Option D uses told, includes the object Rohan immediately after it, and converts the imperative into not to run so fast, which follows the standard pattern. The tense does not need backshifting here because the instruction is of general relevance and the verb run is in base form inside an infinitive. The time relation also does not raise any issues, so the structure is correct.

Why Other Options Are Wrong:
Ram advised Rohan do not run so fast is incorrect because after advised we expect either an object plus to infinitive (advised Rohan to run) or a that clause; the negative imperative is not correctly converted and is missing not to.

Common Pitfalls:
Ram asked Rohan why is he running so fast completely changes the sentence type from command to question and introduces why is he running, which was not present in the original. Ram requested Rohan not to run so fast could be grammatically acceptable but request suggests a polite appeal, whereas said to does not necessarily imply politeness. Exam keys usually choose the most neutral and direct match, which is told plus not to. Therefore it is safer to go with the option that follows the standard transformation rules exactly.

Final Answer:
The correct indirect speech form is Ram told Rohan not to run so fast.