In a UML activity diagram, which symbol represents a task?

In a UML (Unified Modeling Language) Activity Diagram, the symbol used to represent a task (or action/activity) is a round-cornered rectangular box. This shape denotes an individual piece of work or step carried out within the process.

The other symbols represent:

A filled in circle (B): The Initial Node (start of the process).

A diamond shaped box (C): A Decision or Merge Node (a point where a choice is made or paths recombine).

A bullseye (D) (a filled circle inside an outer circle): The Activity Final Node (end of the process flow).

(Reference: BCS Modelling Business Processes / BCS Business Analysis Practice – UML Activity Diagrams)

===========