**Core Concept**
Fibers susceptible to hypoxia are those with high metabolic demands, relying heavily on oxidative phosphorylation for energy production. These fibers are often found in muscles involved in sustained, high-intensity activities.

**Why the Correct Answer is Right**
The correct answer is **Group IIb (IIa)**. These fibers are primarily involved in glycolytic metabolism, relying heavily on anaerobic glycolysis for energy production. As a result, they are more susceptible to hypoxia and fatigue. In contrast, Type I (Group I) fibers, which are primarily oxidative, are more resistant to hypoxia. Type IIa fibers, while less oxidative than Type I, are more resistant to hypoxia than Type IIb fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a distractor, but it's unclear which specific fiber type it refers to. Assuming it's a common fiber type, it's still not the correct answer as we've discussed.

**Option B:** This option is also unclear, but if it refers to a specific fiber type, it's not the correct answer as per our discussion.

**Option D:** This option is also unclear and doesn't directly relate to the correct answer.

**Clinical Pearl / High-Yield Fact**
Remember that muscle fibers can be broadly categorized into two types: oxidative (Type I) and glycolytic (Type II). Oxidative fibers are more resistant to fatigue and hypoxia, making them suitable for sustained, low-intensity activities.

**Correct Answer:** C. Type IIb (IIa) fibers are more susceptible to hypoxia due to their reliance on anaerobic glycolysis for energy production.