In this reported speech item, the direct sentence is I said to you, "He should not be trusted", and you must choose the indirect version that correctly reports this statement with appropriate changes in pronouns and structure. Select the alternative that best expresses the same warning or advice in indirect speech.

Introduction / Context:
This question checks your understanding of reporting a statement that contains the modal verb should, which expresses advice or moral judgement. The direct sentence is I said to you, "He should not be trusted". In indirect speech, said to you is usually changed to told you, and the quoted clause is introduced with that. We must preserve the modality expressed by should, because it represents the speaker's opinion or advice about trusting the person.

Given Data / Assumptions:

• Reporting clause: I said to you.
• Reported clause: He should not be trusted.
• The speaker is giving a warning or piece of advice.
• We need to select the best indirect form from the given options.

Concept / Approach:
When converting direct speech containing should into indirect speech, the modal should usually remains unchanged because it already expresses a kind of past independent modality that is not tied to a specific time frame in the same way as simple present or past forms. The verb said to with an object is normally replaced by told with that plus a clause. Therefore, the correct indirect sentence should have I told you that followed by he should not be trusted. Changing should to cannot or shall not would modify the meaning and is not required by the rules of reported speech.

Step-by-Step Solution:
First, identify the two parts of the direct sentence: the reporting part I said to you and the content He should not be trusted. Second, change said to you into told you, which is the usual reporting verb structure when there is a direct object. Third, introduce the reported clause with that and keep the pronoun he as it is, since the person being discussed does not change. Fourth, keep the modal should because it expresses advice or recommendation and does not need tense backshift. Fifth, combine these elements to form I told you that he should not be trusted.

Verification / Alternative check:
To check, convert the candidate indirect sentence back into direct speech. From I told you that he should not be trusted we can recover I said to you, "He should not be trusted". The meaning and modal force are the same, and the only structural change is the use of told instead of said to and the addition of that. This symmetry confirms that the indirect sentence preserves the original sense of warning or advice.

Why Other Options Are Wrong:
Option A, I said to you do not trust him, is awkward because it does not use that and still tries to keep an imperative structure, which is not the same as the original modal statement.

Option B, I told you that he cannot be trusted, changes should not be trusted to cannot be trusted, which is stronger and states an absolute impossibility rather than a recommendation or judgement.

Option C, I told you he shall not be trusted, uses shall not, which sounds formal and future oriented, and again shifts the meaning away from advice toward a prediction or decision.

Common Pitfalls:
Learners sometimes think that all modal verbs must change in reported speech, but should, could, might, and ought to often remain unchanged because they do not refer directly to a specific time. Another pitfall is to over generalise the use of cannot and shall not in indirect statements, which alters the degree of obligation or possibility expressed by the original sentence. It is essential to respect the meaning encoded by the modal word when converting between direct and indirect forms.

Final Answer:
The indirect sentence that accurately reports the warning while preserving the modal should is I told you that he should not be trusted.