Step 1: Understand the idea in the quote.
The statement says landforms reach a balance, an equilibrium, between forces that resist erosion and forces that drive it. This is an equilibrium or dynamic balance idea.
Step 2: Recall Davis.
Davis is famous for the cycle of erosion, the youth-maturity-old age model, not for the equilibrium idea.
Step 3: Recall Penck.
Penck linked landforms to the rate of uplift versus erosion, a different emphasis.
Step 4: Recall Horton.
Horton is known for the laws of stream ordering and drainage networks, not equilibrium of forces.
Step 5: Recall Gilbert.
G. K. Gilbert proposed that landforms tend toward a balance between resisting and driving forces, the equilibrium concept named in the quote.
Step 6: Choose the answer.
The author is Gilbert (1918), option 3.
\[ \boxed{\text{Gilbert (1918)}} \]