Answer: A. Spruce trees reduce the sunlight available to young pine trees, thus decreasing the environment's carrying capacity for pine trees.
Step-by-step explanation:
Carrying capacity is the maximum number of the members of the population of a species which an ecosystem can sustain in terms of availing the resources. The population size will be affected when the carrying capacity does not meet the demand of the population.
In the given situation, the pine trees requires the sunlight to mature whereas the spruce trees requires the shade to get mature. If the spruce trees are taller than the pine trees they are likely to reduce the available sunlight of the young pine trees. Hence, the environmental carrying capacity for pine trees will decrease in the absence of proper sunlight.