When we convert a direct speech which conveys a command, request or suggestion into indirect speech, we do two things:
(a) The indirect speech is introduced by some verb expressing the command (Eg: commanded/instructed), request (Eg: requested/pleaded) or suggestion (Eg: advised/suggested). The given sentence with blank no.30 contains a suggestion. Hence, the verb, 'advised' is used.
(b) The imperative mood is changed to the infinitive (i.e 'to+present tense form of the verb'). The verb in the speech is: 'take'. Hence, the phrase, 'to take' has to be used.
In indirect speech, words which express nearness (Eg:this) is changed into words that express distance (Eg: that). In the given sentence, the phrase, 'this tincture' in the direct speech is changed into 'that tincture' in indirect speech.
The complete sentence in indirect speech is: 'She, then, advised him to take that tincture to cure his stomach ache'. Option B is correct as the sentence in indirect speech is correctly mentioned.
Option A is incorrect as the past tense form of the verb (took) is used instead of the present tense (take).
Option C is incorrect as the phrase, 'this tincture' has not been changed.
Option D is incorrect as option B is the correct answer.