Final answer:
March is the month that is signed by spelling out the first four letters, being the exception to the rule that months longer than five letters are abbreviated to the first three letters.
Step-by-step explanation:
The question revolves around the unique signing method for the names of months with more than five letters, where only the first three letters are usually spelled out, with one exception that requires four letters. The month that is the exception to this rule and is shown by spelling the first four letters is March. This is because, typically, months that are longer than five letters are abbreviated to the first three letters when spelled in sign language or in short forms. Since June, April, and May are already shorter than or equal to five letters, they wouldn't generally be abbreviated. Therefore, March is the only month among the options provided that is commonly signed by spelling out the first four letters to avoid confusion with other words or signs.