Staements I: `._zX^4` undergoes `2 alpha`-decays, `2 beta`-decays (negative `beta`) and `2 gamma`-decays. As a result, the daughter product is `_(z-2)X^(A-8)`.
Staements II: In `beta`-decay, the mass number decreases by 4 unit and atomic number decreases by `2` unit. In `beta`-decay (negative `beta`), the mass number remains unchanged and atomic number increases by `1` unit. In `gamma`-decay, mass number and atomic number remain unchanged.

Explanation

To solve the problem, we will analyze the given statements step by step.

### Step 1: Understanding the initial state
We start with a nucleus represented as \( _ZX^A \), where \( Z \) is the atomic number and \( A \) is the mass number.

### Step 2: Alpha Decay
In alpha decay, the nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons. This results in:
- The atomic number \( Z \) decreases by 2.
- The mass number \( A \) decreases by 4.

After the first alpha decay:
- New atomic number: \( Z - 2 \)
- New mass number: \( A - 4 \)

After the second alpha decay:
- New atomic number: \( Z - 4 \)
- New mass number: \( A - 8 \)

### Step 3: Beta Decay
In beta decay (specifically negative beta decay), a neutron is converted into a proton, which results in:
- The atomic number \( Z \) increases by 1.
- The mass number \( A \) remains unchanged.

After the first beta decay:
- New atomic number: \( Z - 4 + 1 = Z - 3 \)
- Mass number remains: \( A - 8 \)

After the second beta decay:
- New atomic number: \( Z - 3 + 1 = Z - 2 \)
- Mass number remains: \( A - 8 \)

### Step 4: Gamma Decay
Gamma decay involves the emission of gamma radiation, which does not change the atomic number or mass number. Thus, after two gamma decays:
- Atomic number remains: \( Z - 2 \)
- Mass number remains: \( A - 8 \)

### Step 5: Final Result
After all the decays, the final daughter product is:
- Atomic number: \( Z - 2 \)
- Mass number: \( A - 8 \)

This confirms that Statement I is true.

### Step 6: Analyzing Statement II
Now let's check Statement II:
1. In alpha decay, the mass number decreases by 4 units and atomic number decreases by 2 units. **(True)**
2. In beta decay, the mass number remains unchanged and atomic number increases by 1 unit. **(True)**
3. In gamma decay, mass number and atomic number remain unchanged. **(True)**

Since all parts of Statement II are correct, we conclude that Statement II is also true.

### Conclusion
Both Statement I and Statement II are true, and Statement II correctly explains Statement I.