Final answer:
The plural error in the given sentence is the use of the apostrophe in 'commuter's'. The correct form should be 'commuters' without an apostrophe, as it indicates the plural form of the word. So the corrected sentence would be: 'a year in which approximately 1.5 million commuters took buses and trains to work.'
Step-by-step explanation:
The plural error in the given sentence is the use of the apostrophe in 'commuter's'.
The correct form should be 'commuters' without an apostrophe, as it indicates the plural form of the word.
So the corrected sentence would be: 'a year in which approximately 1.5 million commuters took buses and trains to work.'