The correct option is A Eucalyptus regnans
The largest living tree on earth is Eucalyptus regnans. It is an evergreen tree. Eucalyptus regnans is the tallest of the eucalyptus, growing to 70114.4 m, with a straight, grey trunk and smooth bark except for the rough basal. The flowers are produced in clusters of 915 together, each flower is with a ring of numerous white stamens.
Therefore, the correct answer is option A.