## Core Concept
The Eustachian tube, also known as the auditory tube or pharyngotympanic tube, plays a crucial role in equalizing the air pressure on both sides of the eardrum, which is essential for the proper transmission of sound. It connects the middle ear to the nasopharynx.

## Why the Correct Answer is Right
The Eustachian tube is characterized by its specific anatomy and function. It is approximately 3.75 cm in length, which matches option **a) Size is 3.75 cm**. The tube is indeed partially cartilaginous and partially bony, with the cartilaginous portion making up about 1/3 of its length and the bony portion making up about 2/3, but this description seems to be slightly misplaced in option **b) Cartilagenous 1/3 & 2/3rd bony** as the typical description is the opposite - 1/3 bony and 2/3 cartilaginous. The tube opens during swallowing, which is correctly stated in option **c) Opens during swallowing**. This action is facilitated by the tensor veli palatini muscle, which helps to open the Eustachian tube during swallowing, supporting option **e) Tensor palati helps to open it**. The nasopharyngeal opening of the Eustachian tube is indeed narrower than the tympanic end, which relates to option **d) Nasopharyngeal opening is narrowest**.

## Why Each Wrong Option is Incorrect
- **Option B:** This option suggests a specific combination that might not accurately reflect the correct statements based on the details provided.
- Further detailed analysis on each option is not provided as we directly evaluate the options based on provided and known facts.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that dysfunction of the Eustachian tube can lead to middle ear problems, such as otitis media with effusion. The tensor veli palatini muscle's role in opening the Eustachian tube during swallowing is crucial for maintaining middle ear pressure equality.

## Correct Answer: C.