Final answer:
The comparative phrase 'Cleaner than' is the correct answer because it is used to directly compare the cleanliness of one thing to another, which fits the context of a 'rational word' or rather a comparative word (B).
Step-by-step explanation:
The question asks to identify a rational word, however, based on context, it seems the question is looking for a comparative word that can be used in a comparative context. Among the options provided, 'Cleaner than' is a comparative phrase that is used to compare one thing with another in terms of cleanliness. In contrast, phrases such as 'Too much,' 'Pretty,' and 'As much as' are not strictly used for comparisons in the same way.
Comparative phrases are key in English grammar and are followed by a noun or a pronoun. The phrase 'Cleaner than' is followed by a noun to indicate that something has a greater degree of cleanliness in comparison to something else. For example, 'We can say that a freshly washed car is cleaner than one that hasn't been washed in weeks.'