The correct option is
B Since
The preposition 'from' indicates a point in space at which a journey, motion, or action starts. As there is no point in space indicated by the word, 'Saturday,' option A is inappropriate.
The preposition 'since' is used to indicate an action which is continuing from a particular time in the past until a later time, or until now. The given sentence indicates that the subject (Ritika) has been unwell from Saturday until now. Hence, the appropriate preposition is 'since' as given in option B.
The preposition 'for' is used to show an amount of time or distance. In the given sentence, the word 'Saturday' does not indicate the amount of time that the subject (Ritika) has been unwell. Hence, option C is incorrect.
The preposition 'on' is used to indicate the position of a noun when it is in contact with an upper surface of something. As 'the word Saturday' does not indicate the position (in space) of any noun, option D is inappropriate.