Which type of storage tank is designed to operate at atmospheric or non-pressure?

The correct choice refers to a storage tank designed specifically to operate at atmospheric or non-pressure conditions, which aligns with the characteristics of a horizontal tank. Horizontal tanks are typically used for storing liquids and are designed to withstand the weight of the liquid without the need for additional pressure. This means they are not constructed to hold gases or liquids at elevated pressures, making them suitable for a variety of non-pressurized applications, such as fuel storage or chemical storage.

In contrast, other tank types mentioned, such as pressure tanks and spherical tanks, are specifically designed for handling pressure, which isn't the case for atmospheric tanks. Pressure tanks can withstand significant internal pressures and are often used in applications where liquids need to be stored under various pressures for safety or performance reasons. Low-pressure tanks, while still operating below atmospheric conditions, are not typically considered appropriate for standard atmospheric storage due to their design parameters meant for pressures higher than what is found in a purely atmospheric tank.

Thus, a horizontal tank stands out as the correct choice for a tank intended to function under atmospheric or non-pressure conditions.