**Core Concept**
Muscle hypertrophy is a condition characterized by an increase in muscle mass, often resulting from resistance training or genetic conditions. The molecular adaptations underlying muscle hypertrophy involve changes in the expression and composition of contractile proteins, including myosin heavy chains.

**Why the Correct Answer is Right**
During muscle hypertrophy, there is a shift from the slower-twitch, type I myosin heavy chain (MHC) to the faster-twitch, type II MHC. This change is mediated by the activation of the mTOR signaling pathway, which regulates protein synthesis and muscle growth. The increased expression of type II MHC enhances muscle force production and contraction velocity, contributing to the increased muscle mass.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the change in myosin heavy chain expression during muscle hypertrophy.
* **Option B:** This option is incorrect because it suggests a change in the expression of actin, which is not directly related to muscle hypertrophy.
* **Option C:** This option is incorrect because it implies a decrease in muscle mass, which is the opposite of muscle hypertrophy.

**Clinical Pearl / High-Yield Fact**
A key adaptation in muscle hypertrophy is the increased expression of type II myosin heavy chains, which enhances muscle force production and contraction velocity. This adaptation is mediated by the activation of the mTOR signaling pathway.

**Correct Answer:** B. Type II myosin heavy chain.