Error spotting in a complex English sentence: As you know that the ignorant are easily duped. Decide which part of the sentence is grammatically incorrect or state that there is no error.

Introduction / Context:
This question examines correct clause structure and the use of conjunctions in English. The sentence uses As you know and also includes the conjunction that, leading to a redundancy problem. Error spotting items like this ensure that candidates understand which connectives are necessary and which are unnecessary or incorrect in a given pattern.

Given Data / Assumptions:
- Sentence: As you know that the ignorant are easily duped.
- Part (a): As you know
- Part (b): that the ignorant
- Part (c): are easily duped
- Part (d): No error
We assume that the intended meaning is You know that the ignorant are easily deceived, or As you know, the ignorant are easily deceived.

Concept / Approach:
As you know is already a complete introductory clause and does not require that immediately after it when the next part is a full clause. There are two correct patterns: - You know that the ignorant are easily duped.
- As you know, the ignorant are easily duped.
Combining As you know with that creates an unnatural and incorrect construction. Therefore, the presence of that after As you know is the main error in the sentence, and this is associated with part (a) as framed in the original division.

Step-by-Step Solution:
Step 1: Consider the pattern in part (a) As you know. This can function as an introductory clause that prepares the listener for known information. Step 2: Examine part (b) that the ignorant. The word that typically introduces a noun clause, for example, You know that the ignorant are easily duped. Step 3: When we begin the sentence with As you know, we usually follow it by a comma and then a complete clause, such as the ignorant are easily duped. Step 4: Therefore, either we remove As from As you know and keep that, or we keep As you know and remove that. Step 5: The most natural correction is: As you know, the ignorant are easily duped. Step 6: This shows that the error arises from the combination As you know that, which belongs to the first part as given in the question.

Verification / Alternative check:
We can write two grammatically correct alternatives to confirm our reasoning: - You know that the ignorant are easily duped.
- As you know, the ignorant are easily duped.
Both versions are acceptable and convey the same idea that people who lack knowledge are easily deceived. The original form mixes elements of both and produces an incorrect hybrid. This verification confirms that the redundant conjunction that following As you know is the real error.

Why Other Options Are Wrong:
- Error in part (b) only: Apart from the presence of that, the phrase the ignorant is correct as a collective noun phrase. The core mistake is the pattern As you know that, which is credited to the introductory part.
- Error in part (c) only: Are easily duped is correct; the verb agrees with the plural noun phrase the ignorant and duped is the right past participle of dupe.
- No error in the sentence: This ignores the redundancy created by As you know that and is therefore incorrect.
- Error in more than one part: Only the introductory structure needs to be changed; the rest is fine, so multiple errors is not accurate.

Common Pitfalls:
Many learners feel that more connectives make a sentence sound more formal, so they add that unnecessarily. However, English has fixed patterns such as As you know plus comma, followed directly by a clause. Learners should remember that redundancy in conjunctions can be treated as an error in exam questions. A good strategy is to read the sentence without one of the conjunctions and see if it becomes clearer and more natural.

Final Answer:
Correct option: Error in part (a) only.