Which of the following is a means of measuring primary production?

The means of measuring primary production is the harvested plants in a unit area. Primary production refers to the amount of organic matter produced by photosynthesis in an ecosystem, and one way to measure this is to quantify the amount of plant material produced in a given area over a given time period. The harvested plant method involves clipping or cutting all the above-ground plant material in a defined area, drying and weighing it, and then calculating the total amount of plant biomass produced per unit area. This method provides an estimate of the amount of energy available to support other trophic levels in the ecosystem, including herbivores and carnivores. The other options listed in the question are not measures of primary production, but rather different ecological parameters such as reproductive rate, lifespan, and predator-prey relationships.