"the rain in spain stays mainly on the plain." classify which sound device is being used, and explain your reasoning.

The answer is:  " assonance " .

Explanation:

           The answer is "assonance"— since there are repeated, nearby words with the same vowels that are internal (i.e. not at the beginning of the word, as in "alliteration") within words that are repeated and/or nearby.
        {This is the definition or description of "assonance".}. 

        {Note that "alliteration" is not limited to nearby words with initial consonant sounds —but also refers to nearby words with repeated initial vowel sounds; such as:
   
   "Always avoid annoying alliteration."  }.

    Furthermore, in "assonance", the vowel sounds do NOT have to be in within the FIRST syllable of each of the words—nor do the vowel sounds in repeated, nearby words HAVE to be within the SAME SYLLABLES;  rather, this just happens to be the case in the actual question being asked.
 
     Specifically, in the example provided:

        "The rain in Spain stays mainly on the plain."    ;

          the words:  "rain", "Spain", "mainly", "stays", "mainly", "plain" ;  are words that are very close together with the same vowel sound of the "long 'a' "  ;   providing examples of "assonance".

 Hope this explanation—though lengthy—is of some help!