## Core Concept
The transmission of action potential in myofibrils, specifically within skeletal muscle cells, involves the structures responsible for conducting the electrical impulse that leads to muscle contraction. This process is fundamental to understanding how muscles are activated to contract.

## Why the Correct Answer is Right
The correct answer, **T-tubules (transverse tubules)**, are critical for the transmission of the action potential into the interior of the muscle fiber. T-tubules are specialized channels that penetrate the muscle fiber and allow the action potential to reach the interior, where it can activate the **sarcoplasmic reticulum (SR)** to release calcium ions. This calcium release is essential for initiating muscle contraction through the **sliding filament theory**, where actin and myosin filaments slide past each other.

## Why Each Wrong Option is Incorrect
- **Option A:** While **Sarcoplasmic Reticulum (SR)** is involved in the muscle contraction process by releasing calcium ions, it is not directly responsible for transmitting the action potential.
- **Option B:** **Myofibrils** are the contractile units of the muscle fiber but are not the means by which action potentials are transmitted.
- **Option D:** Although **Sarcolemma** (the muscle fiber membrane) is the site where the initial action potential is generated, the question specifically asks about transmission within myofibrils.

## Clinical Pearl / High-Yield Fact
A key point to remember is that T-tubules are crucial for coordinated muscle contraction, especially in skeletal muscles. Their role ensures that the action potential reaches the interior of the muscle fiber, synchronizing the contraction of the entire fiber.

## Correct Answer: C. T-tubules.