In DataWeave 2.x a user-defined function is declared with the fun keyword, typed parameters enclosed in parentheses, and the = operator before the body. Option B follows that exact grammar and concatenates the literal "PC-", the productCategory string and the itemID converted to String with ++. The return type is inferred automatically, so no explicit type annotation is required.

A. Uses { } and the -> arrow; this is not valid DataWeave function syntax.

C. Uses keyword function, which DataWeave 2.x does not recognise for declarations (must be fun).

D. Declares a var (variable), not a function, and employs ->, so it cannot be invoked as a function.

1. MuleSoft Documentation – DataWeave Reference “Defining Functions”, Example: fun add(a: Number, b: Number) = a + b (v.4.6, docs.mulesoft.com).

2. MuleSoft “DataWeave Language Guide”, Functions section, p.8 (PDF download from Anypoint docs site).