Fill in the blank with a suitable Adverb of frequency by choosing the correct option:

I have _______ told him to improve his handwriting.

often

The correct answer is Option (3) → often

• The sentence "I have _______ told him to improve his handwriting." requires an adverb of frequency, which describes how often an action occurs.
• "Often" is an adverb of frequency that means many times or frequently, making it the most suitable choice.
• "Really" (adverb of degree) and "calmly" (adverb of manner) do not fit the context.
• "Yesterday" is an adverb of time, which does not indicate frequency.

Thus, the correct sentence is: "I have often told him to improve his handwriting."