Final answer:
A knot on a wooden board indicates the former location of a branch on the tree. The stem of a plant is characterized by nodes and internodes, and the growth of branches is related to the shoot apical meristem.
Step-by-step explanation:
A knot in the board shows the location of where a branch once grew out of the tree. Hence, the correct answer is option a) Branch.
The stem of a plant bears the leaves, flowers, and fruits and is characterized by the presence of nodes, which are the points of attachment for leaves or branches, and internodes. When a branch is pruned or naturally falls off, it often leaves behind a distinctive feature in the wood known as a knot.
Additionally, understanding that branches result from the shoot apical meristem producing a potential branch or bud primordium in the axil of each leaf, further helps in justifying why the knot represents the location of a past branch attachment.