Investment is the act of spending money or capital with the expectation of generating income or profits over time. It involves making choices to allocate resources to different assets or projects in order to achieve future financial benefits.

Investment can take various forms depending on the nature and purpose of the expenditure. In the given options, the correct answer is option 'D', which includes all of the following: factory building, machines, and equipment. Let's discuss each of these components in detail:

1. Factory Building:
Investing in a factory building refers to the expenditure made to construct or purchase a physical structure where production or manufacturing activities take place. This could involve buying land, constructing the building, installing utilities, and setting up necessary infrastructure. The factory building provides a dedicated space for the production process and serves as a long-term asset for the business.

2. Machines:
Investing in machines involves purchasing or leasing mechanical devices or equipment that aid in the production or operation of a business. Machines can be used for various purposes, such as manufacturing products, processing raw materials, or automating certain tasks. Examples of machines include assembly lines, conveyor belts, CNC machines, printing presses, and industrial robots. These machines enhance productivity, efficiency, and output, thereby contributing to the growth and profitability of the business.

3. Equipment:
Investing in equipment refers to acquiring tools, devices, or instruments that are necessary for the smooth functioning of a business. Equipment can range from small tools like computers, printers, and office furniture to larger assets like vehicles, heavy machinery, and specialized instruments. These assets enable businesses to perform specific tasks, deliver services, or carry out operations effectively. Investing in equipment helps in improving the overall efficiency, quality, and capacity of the business operations.

By investing in factory buildings, machines, and equipment, businesses can enhance their production capabilities, streamline operations, and increase their potential for generating revenue. These investments are crucial for long-term growth and sustainability. They enable businesses to expand their capacity, meet customer demands, improve product quality, and stay competitive in the market.

In conclusion, investment involves spending on factory buildings, machines, and equipment, as all these components play a vital role in the growth and success of a business. These investments contribute to the overall productivity, efficiency, and profitability of the business operations.