In the sentence "The two children, brother and sister, were onto their way to school", which part contains the grammatical error?

Introduction / Context:
This error spotting question tests your knowledge of correct prepositional phrases used with be on the way. The sentence describes two children, a brother and sister, going to school. You must identify which numbered part of the sentence contains incorrect wording that makes the expression unidiomatic or grammatically wrong.

Given Data / Assumptions:

• Part 1: The two children,
• Part 2: brother and sister, were onto
• Part 3: their way to school.
• Part 4: No Error.
• The intended expression is were on their way to school.

Concept / Approach:
The standard English phrase to indicate that someone is travelling towards a destination is to be on one way to a place. We say They were on their way to school, not onto their way. The preposition onto usually indicates movement to a position on top of something and does not collocate with way in this expression. The rest of the sentence is correct: The two children, brother and sister is a proper appositive phrase, and their way to school is also correct. Thus, the error lies in part 2 with were onto instead of were on.

Step-by-Step Solution:
Step 1: Read the full sentence: The two children, brother and sister, were onto their way to school.Step 2: Recognise the familiar idiom be on the way to somewhere.Step 3: Identify that onto is incorrect in this idiom because it implies movement on top of a surface rather than towards a destination.Step 4: Replace were onto with were on, giving The two children, brother and sister, were on their way to school.Step 5: Conclude that the grammatical error is in part 2.

Verification / Alternative check:
Consider similar sentences: She is on her way to the office, The train is on its way, We are on our way home. In all these examples, the preposition on is used, not onto. If we tried to use onto, such as We are onto our way home, the sentence would sound unnatural and incorrect. Parts 1 and 3 of the question sentence mirror these standard patterns except for the preposition in part 2, confirming that this is where the correction is necessary.

Why Other Options Are Wrong:
Part 1 The two children is a correct subject phrase. The additional apposition brother and sister is correctly placed between commas and does not introduce any grammatical errors. Part 3 their way to school correctly completes the idiom on their way to school aside from the wrong preposition earlier. Part 4 No Error cannot be right because there is a clear and well established idiom that the sentence violates.

Common Pitfalls:
Some learners may be uncertain between on and onto because both involve the idea of position on something. However, adding to in onto usually implies movement onto a surface or object, as in climb onto the roof. When talking about progress towards a destination, the correct phrase is always on the way, not onto the way. Keeping this distinction in mind helps you avoid similar errors in writing and exams.

Final Answer:
The incorrect part is 2, where onto should be replaced by on, so option B is correct.