-  'a' to be used in place of 'the'

The sentence suggests that Mr. Thakur lived in a house that is three hundred years old. However, the correct way to express the age of a house is by using the singular form of "year" as "year" is a unit of time and cannot be pluralized. Hence, the correct sentence would be "in the three-hundred-year-old house".

The correct sentence after correction:

Mr. Thakur lived (1)/in the three-hundred-year-old (2)/ house (3)/ in Mumbai. (4)/ No error

To summarize:

• The error is in Part 2 of the sentence: "in the three-hundred year".
• The correct way to express the age of a house is by using the singular form of "year" as "year" is a unit of time and cannot be pluralized.
• The corrected sentence is "in the three-hundred-year-old house".
• Hence, the answer is option B: 2.