To form a platoon at normal interval, which command should be given?

To form a platoon at normal interval, "Fall in" is the appropriate command to issue. This command instructs the troops to assemble in their designated formation while maintaining specified distances between individuals, known as normal intervals. It sets the expectation for the personnel to be alert and present in an organized manner without requiring additional instructions on how to stand.

The other commands serve different purposes: "Stand ready" is generally used to prepare members for immediate action, often in a more combat-oriented context; "Form up" is a broader term that could refer to mobilizing in any formation, but lacks the precision of "Fall in" regarding intervals; "At ease" allows personnel to relax but is not a command for formation and would not initiate the assembly process. Therefore, the choice to use "Fall in" ensures the platoon is correctly formed and ready for further instructions.