Plantation farming is a type of commercial farming where a single crop is grown on a large scale over an extensive area. This practice is characterized by the cultivation of monoculture crops, which means that only one type of crop is grown. Plantation crops are typically grown in tropical or subtropical regions, where the climate and soil conditions are favorable for their growth.

The correct answer to the question is option 'D', which states that both coffee and rubber are usually grown in plantation farming. Let's explore why this answer is correct:

1. Coffee:
- Coffee is one of the most important plantation crops worldwide. It is primarily cultivated in regions with a tropical climate, such as Brazil, Colombia, Vietnam, and Ethiopia.
- Coffee plants require specific conditions to grow, including a temperature range of 60 to 70 degrees Fahrenheit and a well-drained soil with a pH between 6 and 6.5.
- Coffee beans are harvested from the plants and processed to produce coffee, which is a popular beverage consumed globally.

2. Rubber:
- Rubber is another significant plantation crop that is grown extensively in tropical regions, including countries like Thailand, Indonesia, and Malaysia.
- Rubber trees require a hot and humid climate to thrive, with temperatures between 70 and 95 degrees Fahrenheit and an annual rainfall of around 2000 to 3000 mm.
- The latex sap obtained from rubber trees is collected, processed, and used to produce various rubber products, including tires, gloves, and shoes.

Both coffee and rubber are examples of plantation crops because they are cultivated on a large scale over extensive areas. These crops require specific climate and soil conditions for their successful growth, making them suitable for plantation farming practices.

In conclusion, plantation crops refer to monoculture crops grown on a large scale over an extensive area. Coffee and rubber are two examples of plantation crops commonly grown in tropical regions.