Explanation:

- The Pradhan Mantri Shram Yogi Mandhan Yojana is aimed at providing old age security to workers in the unorganised sector.

- The monthly income threshold for eligibility under this scheme is crucial for identifying the beneficiaries.

- Option 1: Rs10,000 - This option is incorrect. The scheme has a different income limit.

- Option 2: Rs12,000 - This figure is also incorrect for the income threshold.

- Option 3: Rs15,000 - This is the correct option. It accurately reflects the income eligibility limit for the scheme.

- Option 4: Rs20,000 - This exceeds the actual income limit set by the scheme.