Final answer:
The sentence that best combines the ideas using a noun clause is: It is really true that water aids digestion. The noun clause functions as the object of the verb 'is' and provides additional information about the truth of the statement.
Step-by-step explanation:
The sentence that best combines the ideas using a noun clause is:
It is really true that water aids digestion.
In this sentence, the noun clause 'that water aids digestion' is used as the object of the verb 'is.' The noun clause functions as a single unit and provides additional information about the truth of the statement 'It is really true.'
Using a noun clause in this way helps to combine the two ideas into a clear and concise sentence.