Porcupine : Rodent :: Mildew : Fungus

In this analogy, we are given a pair of words: "Porcupine" and "Rodent". We need to find another pair of words that has a similar relationship.

Porcupine is a type of animal, specifically a rodent. A rodent is a group of mammals characterized by continuously growing incisors and a single pair of constantly growing teeth in each of the upper and lower jaws. Examples of rodents include mice, rats, squirrels, and beavers.

Similarly, we need to find a word that represents a specific type of organism in the second pair. The word "mildew" refers to a type of fungus.

Mildew is a type of fungus that often appears as a white or gray powdery substance on surfaces. It typically thrives in warm and humid environments. It can be found on various organic materials such as plants, fabrics, and walls. Mildew can cause damage to these materials and can also pose health risks to humans, especially those with respiratory conditions.

Therefore, the relationship between mildew and fungus is similar to the relationship between porcupine and rodent, as mildew is a specific type of fungus, just as porcupine is a specific type of rodent.

Hence, the correct answer is option (d) Fungus.