The odd one out is option 'D' RAM. Let's analyze each option to understand why.

1. Hard disk:
- A hard disk is a storage device used to store and retrieve digital information.
- It is a non-volatile memory, meaning the data remains even when the computer is turned off.
- It is commonly used in computers and laptops to store the operating system, software, and user files.

2. Floppy disk:
- A floppy disk is a storage medium that was commonly used in the past.
- It is a portable, magnetic storage device that can store a small amount of data.
- Floppy disks are no longer widely used due to their limited storage capacity and the advent of more advanced storage technologies.

3. CD (Compact Disc):
- A CD is an optical storage medium used to store and play back audio, video, and other digital data.
- CDs have a larger storage capacity compared to floppy disks and are commonly used for distributing software, music, and movies.
- They can be read by CD drives or CD players.

4. RAM (Random Access Memory):
- RAM is a type of computer memory that is used to temporarily store data that is actively being used by the computer's operating system and applications.
- It is a volatile memory, meaning the data is lost when the computer is turned off or restarted.
- RAM plays a crucial role in the performance of a computer as it allows for faster access to data compared to other storage devices.

5. None of these:
- This option indicates that none of the given options is the odd one out.
- However, based on the analysis of the previous options, it is clear that RAM stands out as it is a type of memory, while the other options are storage devices.

In conclusion, the odd one out is option 'D' RAM because it is a type of computer memory, whereas the other options are storage devices.