Option 1: All rural women. This option is not the target group of the `Garib Kalyan Rojgar Abhiyan` scheme. The scheme focuses on providing employment opportunities to a specific group, which is not limited to rural women.

Option 2: All returning migrants. This is the correct answer. The `Garib Kalyan Rojgar Abhiyan` scheme aims to benefit the returning migrants who have been affected by the COVID-19 pandemic. The scheme aims to provide employment opportunities and livelihood support to these migrants.

Option 3: All below poverty line households. Though poverty alleviation is an important objective of the scheme, the `Garib Kalyan Rojgar Abhiyan` specifically targets returning migrants and not all below poverty line households.

Option 4: All new migrants to town. This option is incorrect. The `Garib Kalyan Rojgar Abhiyan` scheme focuses on supporting returning migrants, not new migrants to town.

In conclusion, the correct answer is option 2: All returning migrants. They are the primary beneficiaries of the `Garib Kalyan Rojgar Abhiyan` scheme.