Fill in the blank with the most appropriate word. 

A: How is your father’s cold? 

B: ______ it gets worse, we will call the doctor. 

Correct alternative is A) 

Let’s have a look at the 2 sentences: A: How is your father's cold? B: ____ it get worse, we are able to name the medical doctor. Sentence A is query and Sentence B is a solution to that query. Sentence B includes clauses. The 2nd clause states that the medical doctor can be referred to as. We can infer from this that father’s circumstance has now no longer deteriorated to the factor wherein a medical doctor is needed and so the medical doctor has now no longer been referred to as until now. The first clause of Sentence B talks of the circumstance if father’s circumstance receives ‘worse’ then the medical doctor can be referred to as. We want a phrase that might specific this conditionality. Option C containing circumstance-specifying phrase ‘if’ can't be used as it might require the verb as ‘receives’ as a substitute than ‘get.’ Option D can't be used as ‘unless’ is utilized in poor sentences. Option C receives dominated out as ‘might’ is used while speaking approximately an occasion that took place withinside the past. Hence, Option A or ‘should’ is the appropriate answer. ‘Should’ is utilized in conditional clauses expressing opportunities and suppositions.