Final answer:
It is true that hyphens are used when letters and numbers are combined to designate military rank, to clearly denote the rank and title together.
Step-by-step explanation:
The statement "Use a hyphen when letters and numbers are used to designate military rank" is true. When writing out the rank and title of military personnel, especially in formal or official documents, a hyphen is typically used between the letters that signify the military branch and the numbers indicating the rank. For example, an Army captain might be designated as O-3, and with a hyphen, it would correctly appear as O-3 Captain. The hyphen in this context helps to clarify the relationship between the parts of the designation, ensuring both are read together as a whole.