Answer:
D. An arrow.
Explanation:
A flow chart can be defined as a diagrammatic representation or a step-by-step chart, representing workflow.
In the given flow diagram, the symbol that should be used is an arrow. An arrow is a symbol, that is used in the flow charts as a 'connector'. It helps to show the direction of the flowing chart. An arrow, in the flowing chart, helps the viewer as guidance.
So, the correct answer is option D.