Final answer:
The only legal identifier among the given options is "Five_&_TenConstant". It follows the rules of a legal identifier in programming languages.
Step-by-step explanation:
The legal identifiers in programming languages are words that are used to name variables, functions, or other entities in a program. In the given options, the only legal identifier is "Five_&_TenConstant". This identifier follows the rules of a legal identifier in most programming languages.