In the following sentence, one part contains a grammatical or idiomatic error. Identify the part of the sentence that has the error. My elder son deals with (1) / spare parts and manages (2) / his bread and butter. (3) / No error (4).

Introduction / Context:
This error spotting question checks your understanding of English idioms, specifically the common phrase earn one's bread and butter. The sentence describes how someone supports himself financially. Your task is to identify which numbered part contains incorrect or non-idiomatic English, even though the individual words may look familiar.

Given Data / Assumptions:
Complete sentence: My elder son deals with (1) / spare parts and manages (2) / his bread and butter. (3) / No error (4).
Part 1: My elder son deals with.
Part 2: spare parts and manages.
Part 3: his bread and butter.
We must decide which part contains the main error, or choose No error if all parts are correct.

Concept / Approach:
In idiomatic English, we say someone earns his bread and butter, meaning he earns his basic livelihood or income. The verb manage is not normally used with bread and butter in this fixed expression. While deals with spare parts is acceptable as a description of his business, manages his bread and butter is awkward and unidiomatic. The approach is to identify which chunk breaks the recognised idiom.

Step-by-Step Solution:
Step 1: Examine part 1: My elder son deals with. The phrasal verb deals with can mean is engaged in a particular business or handles a certain type of product, which fits well with spare parts. Step 2: Combine part 1 and part 2: My elder son deals with spare parts and manages. This still makes sense grammatically, with manages leading into an object. Step 3: Look closely at part 3: his bread and butter. Together with manages, it forms manages his bread and butter, which is not the idiomatic expression used in English. Step 4: The usual phrase is earns his bread and butter or makes his bread and butter from spare parts. The use of manages here is therefore incorrect from an idiomatic standpoint. Step 5: Parts 1 and 2 do not contain clear grammatical or idiomatic errors, so the error must be in part 3. Step 6: Therefore, part 3 is the answer to mark as containing the error.

Verification / Alternative check:
Rewrite the sentence correctly: My elder son deals in spare parts and earns his bread and butter from that business. Comparing this with the original shows that the only element needing change for natural English is manages his bread and butter. That part corresponds to segment 3, confirming your choice.

Why Other Options Are Wrong:
Part 1, deals with, is acceptable in many exam contexts, though deals in would also be fine; examiners normally treat deals with spare parts as correct usage for business dealings.
Part 2, spare parts and manages, simply continues the thought and is not itself incorrect; the problem only appears once we see the object in part 3.

Common Pitfalls:
Students sometimes look for very small grammatical errors and overlook idiomatic usage. However, many exam questions specifically test whether you can recognise fixed expressions like earn one's bread and butter or make a living. When you see such an expression altered awkwardly, that is often the error the exam setter wants you to find.

Final Answer:
The error is in part 3, which incorrectly uses manages his bread and butter instead of the idiomatic earns his bread and butter.