In the following error spotting question, identify which part of the sentence contains a grammatical mistake: One of the hazard of swimming (1) in the ocean is an unexpected (2) encounter with a jellyfish. (3) No error (4).

Introduction / Context:
This question is an error spotting item that tests your understanding of number agreement between subject and noun phrases. Competitive exams often use sentences of this type to check whether candidates can correctly use structures like "one of the" followed by a plural noun, as well as whether they can keep the rest of the sentence grammatically sound. Here, you must decide which numbered part of the sentence includes a grammatical error, or whether the sentence is error free.

Given Data / Assumptions:

• The sentence is divided into four parts, labelled (1), (2), (3), and (4).
• Part (1): One of the hazard of swimming.
• Part (2): in the ocean is an unexpected.
• Part (3): encounter with a jellyfish.
• Part (4): No error indicates that the sentence has no mistake.
• We assume standard exam level British or international English rules.

Concept / Approach:
When English uses the pattern "one of the", the noun that follows must be plural, because it refers to one item taken from a group of many. Thus, we say "one of the hazards", "one of the problems", or "one of the reasons". If the noun after "one of the" appears in singular form, as "hazard", the structure is incorrect. The rest of the sentence describes a specific type of danger related to swimming in the ocean, which is grammatically sound when viewed separately.

Step-by-Step Solution:
Step 1: Focus on part (1): "One of the hazard of swimming". The core pattern is "one of the hazard". Step 2: Recall the rule that the noun after "one of the" must be plural, so we expect "one of the hazards". Step 3: Recognise that "hazard" in the singular breaks this rule and should be corrected to "hazards". Step 4: Check part (2): "in the ocean is an unexpected". This correctly links to the subject phrase and leads into the complement. Step 5: Check part (3): "encounter with a jellyfish." This is a natural and grammatically correct noun phrase. Step 6: Part (4) claims that there is no error, but we have already found an error in part (1), so part (4) cannot be correct. Step 7: Conclude that the only error lies in part (1).

Verification / Alternative check:
Rewrite the corrected sentence: "One of the hazards of swimming in the ocean is an unexpected encounter with a jellyfish." This version sounds fully natural and meets the standard rule about "one of the" followed by a plural noun. No other part requires any change. The verb "is" correctly agrees with the singular phrase "one of the hazards" because the real grammatical subject is "one". This confirms that there is exactly one error and that it is in part (1).

Why Other Options Are Wrong:
Part (2) is correct because "in the ocean is an unexpected" smoothly connects location and complement. Part (3) correctly completes the idea with "encounter with a jellyfish". Part (4), which stands for "no error", is wrong because we have identified a definite mistake earlier in the sentence. Therefore, none of the other parts can be marked as containing the error.

Common Pitfalls:
Many learners focus on the later parts of a sentence and ignore the apparently simple opening structure. Another common mistake is to think that because "one" is singular, the following noun can also be singular. Remember that "one of the" always selects a single item from a plural set, which is why the noun must be plural even though the verb agrees with "one". Keeping this pattern in mind will help you avoid similar errors in other questions.

Final Answer:
The error is in Part (1), because "hazard" should be plural as "hazards" after the phrase "one of the".