According to the Adobe Commerce documentation, the output of a GraphQL mutation is declared by

specifying the type of the data returned by the mutation. The type can be either a scalar type (such

as String, Int, Boolean, etc.), an object type (such as Cart, Product, Customer, etc.), or a union type

(such as SearchResult, which can be either Product or Category). A union type is used when the

mutation can return more than one possible type of data, depending on the input or the logic of the

mutation. In this case, the mutation can return either a Cart type or a ConfigurableProduct type,

depending on the number of variants of the configurable product. Therefore, the output of the

mutation should be declared as a union type that includes both Cart and ConfigurableProduct types.

Option B is the only option that correctly declares a union type using the pipe symbol (|) to separate

the possible types. Option A and Option C are incorrect because they use brackets ([ ]) and curly

braces ({ }), which are used for declaring list types and input object types, respectively.

Reference:

GraphQL API - Adobe Inc.

Schema language with GraphQL | Adobe Commerce