The correct answer would be "Daryl received two 3's as scores on his paper from the readers, but Sheila received two 6's."
When placing a number in a sentence before an S, it is to always have an apostrophe. Along with this, this is the only sentence that uses the correct form of "readers", since in this sentence, "readers" is being used as a plural noun, not a possessive noun.
Possessive nouns contain an apostrophe to show that something belongs to someone else.
Ex: That cat is her's.
The possessive noun is "her's" because it's showing that she is the owner of the cat.
As for plural tense, it speaks for more than one person or object.
Ex: The cats scrambled up the tree.
The plural noun is "cats" because it's referring to more than one cat.
