The acronym SRO is used in the capital market to refer to various market participants. The correct expansion of SRO is "Self Regulatory Organisations".

Option 1: Self Regulatory Organisations - This option correctly expands the acronym SRO. Self Regulatory Organisations are entities that have been granted the authority to regulate and oversee specific aspects of the capital market. They are responsible for setting and enforcing rules and regulations to maintain fair and transparent trading practices.

Option 2: Small Revenue Operators - This option does not correctly expand the acronym SRO. Small Revenue Operators refers to businesses or individuals who generate small amounts of revenue.

Option 3: Securities Roll-back Operators - This option does not correctly expand the acronym SRO. Securities Roll-back Operators does not have a defined meaning in the context of the capital market.

Option 4: Securities Regulatory Organisations - This option does not correctly expand the acronym SRO. While the term "Securities Regulatory Organisations" is similar to the correct expansion, it is not the accurate expansion of the acronym SRO.

Therefore, the correct answer is option 1, Self Regulatory Organisations.