Let `M_(1)` and `M_(2)` be the masses of the nuclei `""_(1)H^(2)` and `""_(2)H^(4)` respectively. Aslo let `m_(p)` and `m_(n)` be the masses of proton and neutron respectively.

Options

A. `m_(p)+m_(n)ltM_(1)`
B. `2[m_(p)+m_(n)]gtM_(2)`
C. `M_(2)=2M_(1)`
D. `M_(2)lt2M_(1)`

Answer

To solve the problem, we need to analyze the masses of the nuclei \( ^2H \) (deuterium) and \( ^4H \) (helium-4) in relation to the masses of protons and neutrons.

### Step-by-Step Solution:

1. **Identify the Composition of Nuclei**:
- The nucleus of \( ^2H \) (deuterium) consists of 1 proton and 1 neutron.
- The nucleus of \( ^4H \) (helium-4) consists of 2 protons and 2 neutrons.

2. **Express the Mass of Deuterium**:
- The mass of deuterium (\( M_1 \)) can be expressed as:
\[ M_1 = m_p + m_n \]
- Here, \( m_p \) is the mass of the proton and \( m_n \) is the mass of the neutron.

3. **Analyze the First Statement**:
- The first statement claims \( m_p + m_n < M_1 \).
- Since \( M_1 \) is defined as \( m_p + m_n \), this statement is **false**.

4. **Express the Mass of Helium-4**:
- The mass of helium-4 (\( M_2 \)) can be expressed as:
\[ M_2 = 2m_p + 2m_n \]

5. **Analyze the Second Statement**:
- The second statement claims \( 2m_p + m_n > M_2 \).
- Since \( M_2 = 2m_p + 2m_n \), this statement is also **false**.

6. **Analyze the Third Statement**:
- The third statement claims \( M_2 = 2M_1 \).
- If we substitute \( M_1 \):
\[ M_2 = 2(m_p + m_n) = 2m_p + 2m_n \]
- This statement is **true**.

7. **Analyze the Fourth Statement**:
- The fourth statement claims \( M_2 < 2m_n \).
- Since \( M_2 = 2m_p + 2m_n \), this statement is also **false** because \( M_2 \) cannot be less than \( 2m_n \) when \( m_p \) is positive.

### Conclusion:
The only true statement is that \( M_2 = 2M_1 \). Therefore, the correct answer is that the second statement is true.