Which of the following statement is//are correct?

Options

A. A diode is a combination
of `p`-and `n`-type
semiconductors which is
used as a rectifier.
B. Transistors are sandwich semiconductors of the type `pnp` or `npn` which are used to detect or amplify radio or audio signals.
C. Monoxides of transition metals, all of which posses `NaCl` structures, show very large variations in electrical properties.
D. `ReO_(3)` has the conductivity as well as appearance like that of copper.

Explanation

To determine which statements are correct, we will analyze each option one by one.

### Step 1: Analyze Option 1
**Statement:** A diode is a combination of p and n type semiconductor which is used as a rectifier.

**Analysis:**
- A diode is indeed formed by the junction of p-type and n-type semiconductors.
- Its primary function is to allow current to flow in one direction (rectification).

**Conclusion:** This statement is **correct**.

### Step 2: Analyze Option 2
**Statement:** Transistors are sandwiched semiconductors of type PNP or NPN type and they are used to detect and amplify radio or audio signals.

**Analysis:**
- Transistors can be of two types: PNP and NPN.
- They are used for amplification and switching, which includes detecting and amplifying audio and radio signals.

**Conclusion:** This statement is **correct**.

### Step 3: Analyze Option 3
**Statement:** Mono oxides of transition metals which have NaCl type structure show very large variation in electrical properties.

**Analysis:**
- Transition metal mono oxides (like TiO, FeO, NiO, CuO) do exhibit a range of electrical properties.
- Their electrical behavior can vary significantly due to differences in their electronic structure and bonding.

**Conclusion:** This statement is **correct**.

### Step 4: Analyze Option 4
**Statement:** Rhenium oxide has the conductivity as well as appearance like that of copper.

**Analysis:**
- Rhenium oxide does exhibit conductivity characteristics similar to copper.
- It also has a metallic appearance, which can be compared to that of copper.

**Conclusion:** This statement is **correct**.

### Final Conclusion:
All four statements are correct.

### Summary of Steps:
1. **Option 1:** Verified the definition and function of a diode.
2. **Option 2:** Confirmed the types and functions of transistors.
3. **Option 3:** Analyzed the electrical properties of transition metal mono oxides.
4. **Option 4:** Checked the characteristics of rhenium oxide.