Silly sally sang a silly summer song. What type of figurative language is this sentence?

Alliteration.

Explanation:

Alliteration is the use of the same sounds or words or letter in sequence in a sentence. The occurrence of the initial consonant letter in close sequence, in successive or closely associated syllables makes up an alliteration. This type of figurative language is frequently used by writers n poetry and other literary works. In the sentence "Silly Sally sang a silly summer song", the initial letters in the words all begin with the consonant letter "s", a sure sign of alliterative technique. So, this sentence is an alliteration.