Explanation:
In a Cathode Ray Oscilloscope (CRO), the horizontal deflection plates are placed vertically. This means that the plates are oriented in a vertical direction.

Reasoning:
The placement of the horizontal deflection plates in a CRO is determined by the purpose they serve in the operation of the instrument. The horizontal deflection plates are responsible for controlling the movement of the electron beam horizontally across the screen of the CRO.

When a voltage is applied to the horizontal deflection plates, it creates an electric field that interacts with the electrons in the beam. This electric field causes the electrons to be deflected horizontally, resulting in the movement of the electron beam across the screen.

Placement of Horizontal Deflection Plates:
The horizontal deflection plates are placed vertically in a CRO for the following reasons:

1. Perpendicular to Vertical Deflection Plates:
The vertical deflection plates of the CRO are placed horizontally. This arrangement allows the electron beam to be deflected both horizontally and vertically, resulting in the formation of a two-dimensional display on the screen.

2. Efficient Deflection of Electron Beam:
Placing the horizontal deflection plates vertically allows for efficient deflection of the electron beam horizontally. The vertical orientation of the plates ensures that the electric field created by the applied voltage interacts effectively with the electrons in the beam, causing them to be deflected horizontally.

3. Compact Design:
Placing the horizontal deflection plates vertically helps in achieving a compact design of the CRO. By arranging the plates vertically, the overall size of the instrument can be minimized, making it more portable and convenient to use.

Conclusion:
In conclusion, the horizontal deflection plates of a CRO are placed vertically. This arrangement allows for efficient deflection of the electron beam horizontally and helps in achieving a compact design of the instrument.