Which of the following are compounds ?
(i) `Co` (ii) `No` (iii) `NO` (iv) `CO`

Options

A. (i) and (ii)
B. (ii) and (iii)
C. (iv) and (iii)
D. (ii) and (iv)

Answer

To determine which of the given options are compounds, we need to understand the definition of a compound. A compound is a substance formed when two or more different elements chemically bond together in a fixed ratio.

Let's analyze each option:

1. **(i) Co**: This is the chemical symbol for cobalt, which is an element. Since it is made of only one type of atom, it is not a compound.

2. **(ii) No**: This appears to be a typographical error. If it is intended to refer to "No" as an element, it could be interpreted as nobelium, which is also an element. Therefore, it is not a compound.

3. **(iii) NO**: This is the chemical formula for nitric oxide, which is formed from nitrogen (N) and oxygen (O). Since it consists of two different elements, it is a compound.

4. **(iv) CO**: This is the chemical formula for carbon monoxide, which is formed from carbon (C) and oxygen (O). Since it consists of two different elements, it is also a compound.

Now, summarizing the findings:
- Co: Not a compound (element)
- No: Not a compound (element)
- NO: Compound (nitric oxide)
- CO: Compound (carbon monoxide)

Thus, the compounds among the given options are **(iii) NO and (iv) CO**.

### Final Answer:
The compounds are: **(iii) NO and (iv) CO**.