The question asks us to identify the part of the sentence that contains a grammatical error.

Grammar Error in Conditional Sentence

Let's examine the sentence: "If I was you / (I)I would not/ (II)ignore the warning / (III)at any cost. (IV)"

The error lies in the first part, labeled (I) "If I was you". This type of sentence expresses a hypothetical or counterfactual situation – something that is not true in reality. For such situations, we use the subjunctive mood in English grammar.

Understanding the Subjunctive Mood

• The subjunctive mood is used for wishes, suggestions, demands, and hypothetical or contrary-to-fact situations.
• In conditional sentences expressing unreal or hypothetical situations (often called second conditional), the past subjunctive form is used.
• For the verb 'to be', the past subjunctive form is 'were' for all subjects (I, you, he, she, it, we, they).
• Therefore, the grammatically correct way to express this hypothetical situation is: "If I were you", not "If I was you".

Analysis of Sentence Parts

• Part (I) "If I was you": Contains the error. It should be "If I were you".
• Part (II) "I would not": This part is grammatically correct, correctly following the hypothetical clause with the conditional mood.
• Part (III) "ignore the warning": This part is also grammatically correct.
• Part (IV) "at any cost.": This adverbial phrase is correctly placed and grammatically sound.

So, the error is located in the first part of the sentence.

Corrected Sentence

The corrected sentence would read: "If I were you, I would not ignore the warning at any cost."

Based on this analysis, the part containing the error is (I).