What type of weed control is involved in planting seedlings at appropriate spacing and replacing those that die?

The correct choice involves cultural control, which encompasses practices that enhance crop health and reduce pest problems through management of the growing environment. Planting seedlings at appropriate spacing helps to ensure that each plant has access to necessary resources such as light, water, and nutrients. This method can contribute to the overall vigor of the plants, making them more resilient to pests and diseases.

Replacing seedlings that die is also a part of cultural control, as it maintains the density of the planting, ensuring that the ecological balance is preserved and that competitive pressures from weeds are minimized. This practice improves the potential for healthy plant growth and ultimately increases crop yields by maintaining effective plant coverage.

Other control methods such as mechanical, chemical, and biological control do not focus on the direct management of plant spacing and the replacement of weakened plants but rather involve the use of physical tools, chemical herbicides, or living organisms to manage pest populations. Therefore, the emphasis on cultivation techniques and environmental optimization makes cultural control the appropriate answer in this context.