What is the Spanish term for "at one"?

The Spanish term for "at one" is "a la una." In Spanish, when referring to specific times, the phrase "a la" is used to indicate the hour for feminine nouns, which includes "una" (one) since it is feminine. Therefore, when you want to say "at one," you would use the phrase "a la una."

The other options represent different times or aspects of time which do not translate directly to "at one." "A las dos" means "at two" and is used for plural hours, "y cuarto" means "quarter past" and refers to the time being a quarter after any hour, while "y media" means "half past" and indicates a time that is thirty minutes past any hour. Each of these phrases serves a different function in discussing time in Spanish, making "a la una" the correct choice for "at one."