Final answer:
The correct rule to apply for the misspelled word 'easyly' is Rule D, which instructs to drop a final 'e' when adding an ending beginning with a vowel, making the correct spelling 'easily'.
Step-by-step explanation:
The correct spelling rule to use for the underlined word in the statement "Setbacks don’t discourage me. I don’t give up easyly." is Rule D: Drop a final e when adding an ending that begins with a vowel. The correct spelling of the underlined word should be 'easily', which is derived from the base word 'easy' plus the suffix '-ly'. You do not need to apply the rule of doubling the final consonant or changing 'y' to 'i', and the 'i before e except after c' rule is not applicable in this case. This is because 'easily' follows the Final Deletion Rule, where the final 'e' from 'easy' is dropped when the suffix beginning with a vowel ('-ly') is added.