Fill in the blank with a suitable Adverb of frequency by choosing the…

The correct answer is Option (3) → often

An adverb is a word that tells how, when, where or how much something happens. It can also describe an adjective or another adverb, for example she sings sweetly, very hot.

Complete sentence: I have often told him to improve his handwriting. The speaker has told him many times to improve his handwriting, so "I have often told him".

Option	What it tells	Example
(1) really	Really is an adverb of degree or emphasis meaning truly, very. It does not tell how often.	The film was really exciting.
(2) calmly	Calmly is an adverb of manner meaning in a quiet, peaceful way. It tells how, not how often.	She answered the angry customer calmly.
(3) often (answer)	Often is an adverb of frequency meaning many times. It tells how often something happens. With a helping verb it stands after the helping verb (have often told).	We have often visited the old fort.
(4) yesterday	Yesterday is an adverb of time. A finished past time cannot be used with the present perfect "have told".	I met him yesterday.

Contrast: often answers "how often?". Calmly answers "how?", really "how much?" and yesterday "when?".