An is a phrase formed from the first letters of words in a set phrase or series of words
a. acronymic sentence
c. basic acronym
b. acronymic abbreviation
d. none of these

The only one that seems to fit the description is "a. Acronymic sentence".

A good example of this is P.E.M.D.A.S., which tends to be remembered as "Please excuse my dear aunt Sally", which we know is really "parentheses, exponents, multiplication, division, addition and subtraction".  Also, as a side note, I just took this quiz, this is the right answer.

no se ingles amiga sorry solo estoy ganando puntos haha