Question

how do plants primarily obtain carbon? by absorbing it from water through their roots from the soil by absorbing it from the atmosphere during photosynthesis

Explanation:

Plants perform photosynthesis, during which they take in carbon - dioxide ($CO_2$) from the atmosphere. Carbon - dioxide is a source of carbon for plants. They do not primarily obtain carbon from water, from soil through roots in significant amounts for carbon needs, or by consuming other plants for their main carbon source.

Answer:

By absorbing it from the atmosphere during photosynthesis