The correct option is C Often
An adverb is a word or set of words that modify verbs, adjectives, or other adverbs. They answer questions such as how, when, where, why, how often, how much etc.
In the given sentence, the question that needs to be asked is "when does he/she play badminton with his/her sister?"
Today, soon and Now are incorrect words to fill in the blank.
'Today' is an adverb as it answers the question when but in the given sentence it is incorrect because it always talks about a specific day and is not used to denote a general day.
'Soon' is an adverb used to indicate something that happens in or after a short time.
'Now' is an adverb that is used to denote something that is happening in the present and cannot be used in the sentence as it is not talking about the present.
Therefore, options A, B and C are incorrect.
The correct answer is option C) often which indicates the frequent occurrence of an event.