The correct answer is sunlight.
Plants compete for all the listed things. However, in contrast to most animals, they strongly compete for sunlight.
Sunlight is one of plants' defining needs, hence their different positions in a forest, grassland or a meadow.
Plants need the sunlight to grow as it stimulates the production of auxins, the growth hormones.
Some plants need more light than others, the adequate amount of sunlight is judged through the photosensitive molecules in their leaves.
If a sun-loving plant finds itself in a shadowy place, the sensors will tell the plant to elongate, causing the plant to grow upwards towards the sun.
