Metabolically active cells, of the adult animal's heart, are remain in which phase ?

Options

A. S-phase
B. `G_(0)` phase
C. `G_(2)` phase
D. M phase

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the phase in which the metabolically active cells of an adult animal's heart remain.

2. **Identifying the Phases**: The phases mentioned are:
- S phase (Synthetic phase)
- G0 phase (Resting phase)
- G2 phase (Second growth phase)
- M phase (Mitotic phase)

3. **Analyzing the G0 Phase**: The G0 phase is also known as the resting phase. In this phase, cells are metabolically active but do not divide. This phase is crucial for cells that have exited the cell cycle but still perform their functions.

4. **Understanding the Role of Heart Cells**: The cells in the heart, such as cardiomyocytes, are highly specialized and need to maintain metabolic activity to support heart function. However, they do not frequently divide.

5. **Conclusion**: Since the heart cells are metabolically active but not proliferating, they remain in the G0 phase.

6. **Final Answer**: The metabolically active cells of the adult animal's heart remain in the G0 phase.