Final Answer:
To access the Build an Atom simulation, you should click the Start button (Option A).
Step-by-step explanation:
To open the Build an Atom simulation, follow these steps:
a) Click the Start button to initiate the simulation. This is the first action to engage with the simulation.
b) Explore the Settings tab to customize and adjust any parameters according to your preferences.
c) Access the Build Molecule option, allowing you to construct molecules within the simulation environment.
Therefore, the correct option is A, as clicking the Start button is the initial step to begin the Build an Atom simulation.