The answer is A.
In a forest, especially a rainforest, trees must grow rapidly to reach the sun at the canopy. In order to be competitive, they grow tall without as wide as trees in moderate climates.
Most trees in these tropical regions have straight trunks with no branches or leaves until they reach the canopy layer. Below this layer there is very little sunlight and trees have adapted to growing branches and leaves where sunlight can be obtained.
Most trees in the rainforest grow rapidly to escape the darkness of the forest floor to reach the needed sunlight of the canopy. When a gap in the canopy appears, for example due to a fallen tree, the small trees are capable of a growth surge in order to take advantage of the opportunity for sunlight.
