1. Identification of Error: Parallelism error and multiple instances of redundancy (tautology).
2. Grammatical Rule: Parallelism requires that elements in a series or comparison (e.g., "not merely X but also Y") must be in the same grammatical form. Redundancy occurs when words or phrases express the same meaning multiple times, making the expression unnecessarily wordy or illogical.
3. Option Elimination:
A. Incorrect – Contains both the parallelism error ("at increasing" vs. "to consequently return back") and several redundancies ("consequently return back," "state of prior financial stability," "future outcome of lasting economic solvency").
B. Incorrect – While it fixes the parallelism ("to increase" vs. "to consequently return") and "return back," it retains significant redundancy ("consequently return to a state of prior financial stability," "future outcome of lasting economic solvency").
C. Correct – Establishes correct parallelism ("to increase," "to return," "to ensure" are all infinitive phrases following "not merely... but also...") and eliminates most redundancies. "Return to financial stability" concisely replaces "consequently return back to a state of prior financial stability," and "ensure lasting economic solvency" streamlines "ensuring a future outcome of lasting economic solvency," with "lasting" providing a defensible emphasis on duration for solvency.
D. Incorrect – While it attempts to fix parallelism by using gerunds ("at increasing" and "returning"), it retains some redundancy ("prior financial stability," "lasting economic solvency") and the construction "not merely at increasing... but also returning... and ensuring" is less elegant and effective than the infinitive parallel structure in option C.