(C) "helps students to become better spellers"
The usage of the article "a" (as well as the word "speller") is improper since it suggests a singularity that conflicts with the plural noun "students." Only C fixes this flaw without creating any more ones.
One is singular in this context. Plural refers to multiples. A noun may typically be made plural by simply adding the letter "s". There are several irregular nouns that add, nevertheless.
The letters at the end of the word serve as the basis for the rules for writing multiple nouns.
Here's another question with an answer similar to this about singularity and plurality: https://brainly.com/question/2516666
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