Fill in the blank with the most appropriate word.
Your English is really improving but mine ________.
A verb is used to express an occurrence, action, or state. It is the grammatical centre of the predicate in a sentence.
An auxiliary verb is a verb which gives is used in the formation of tenses, moods, tones, etc.
Option C is correct as the subject is singular and is really improving indicates the use of present tense.
The other options are wrong as no other tense other than present continuous tense can be used here.
The correct answer is C) isn't