According to RESTful API design principles, a URL path identifies a resource. The path /api/patients refers to the collection of all patient resources. Query parameters, which appear after the ? in a URL, are used to filter, sort, or paginate the collection. Therefore, the query parameter ?year=2021 acts as a filter on the patients collection. The standard and expected behavior for this endpoint is to return a list of all patients that match the criteria of being from the year 2021.

A. Patient with id 2021: Retrieving a single resource by its unique identifier is conventionally done using a path parameter, for example: /api/patients/2021.

C. No patients: While this is a possible outcome if no patients match the filter, it is not the intended function of the query parameter itself.

D. All patients: This would be the expected response for a request to the base collection URI /api/patients without any filtering query parameters.

1. MuleSoft Documentation, "Designing APIs". This guide covers RESTful principles, including the use of resource paths for collections and query parameters for filtering those collections. (docs.mulesoft.com/api-manager/2.x/designing-your-api)

2. RAML 1.0 Specification, "Resource". The specification, which MuleSoft heavily utilizes, details how query parameters are defined and used to "filter the collection of resources". (github.com/raml-org/raml-spec/blob/master/versions/raml-10/raml-10.md#query-parameters)