Which of the Following Best Explains 'External Sustainability Standards

Which of the following best explains 'external sustainability standards'? 

A)  These are used the general codes of conduct and business values of the organization and need to be signed by the supplier. 
B)  These are used to allow procurement professional to check the degree to which suppliers comply with their sustainability requirements through self-assessment and/or supplier sustainability audits. 
C)  These are used to ensure that corrective actions are put in place by suppliers. 
D)  These are used to inform and are based on external standards and norms.