Thermodynamics mainly deals with:

Options

A. interrelation of various forms of energy and their transformation from one form to another
B. energy changes in the processes which depends only on initial and final states of the microscopic system containing a few molecules
C. how and at what rate these energy transformations are carried out
D. the system in equilibrium state or moving from one equilibrium state to another equilibrium state

Explanation

To determine what thermodynamics mainly deals with, let's analyze the given options step by step.

### Step-by-Step Solution:

1. **Understanding Thermodynamics**:
- Thermodynamics is a branch of physical science that deals with the relationships between heat and other forms of energy. It involves the study of energy transformations and the laws governing these transformations.

2. **Analyzing Option A**:
- **Option A** states: "Interrelation of various forms of energy and their transformation from one to another."
- This is correct. The first law of thermodynamics, which states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q) plus the work done on the system (W), supports this. Thus, thermodynamics indeed deals with the interrelation of various forms of energy.

3. **Analyzing Option B**:
- **Option B** states: "Energy changes in the processes which depend only on initial and final states of microscopic systems containing a few molecules."
- This is incorrect. Thermodynamics applies to both macroscopic and microscopic systems, and the energy changes can depend on various factors, not just the initial and final states. Therefore, this option does not accurately represent thermodynamics.

4. **Analyzing Option C**:
- **Option C** states: "How and at what rate these energy transformations are carried out."
- This is also incorrect. The rate of energy transformations is studied in chemical kinetics, not thermodynamics. Thermodynamics focuses on the state functions and energy changes rather than the rates of processes.

5. **Analyzing Option D**:
- **Option D** states: "The system in equilibrium state or moving from one equilibrium state to another equilibrium state."
- This is correct. Thermodynamics examines systems in equilibrium and the conditions under which they can move from one equilibrium state to another. Concepts such as spontaneity and Gibbs free energy (ΔG) are relevant here.

### Conclusion:
Based on the analysis, the correct options that thermodynamics mainly deals with are:
- **Option A**: Interrelation of various forms of energy and their transformation from one to another.
- **Option D**: The system in equilibrium state or moving from one equilibrium state to another equilibrium state.

### Final Answer:
Thermodynamics mainly deals with **Option A and Option D**.

---