Answer:
Option A. 288 in.
Explanation:
Let the tree is h inch tall and shadow of the tree is 168 in.
At the same time a 36 inch yardstick casts a shadow of 21 inches.
Then we have to find the length "h" of the tree.
Both the triangles in the figure attached will be similar.
So by similar triangles property

h = 36×8 = 288 in.
Therefore Length of the tree is 288 in.
Option A. is the correct option.