Step 1: Identify the paradox that needs to be resolved.
The paradox is that two seemingly contradictory things happened at the same time: the number of retail customers (a supposed indicator of health) decreased significantly, while the bank's value (share price) increased dramatically. A good explanation must resolve this contradiction by showing how these two events can logically coexist.
Step 2: Analyze each option to see if it resolves the paradox.
(A) Being cleared of charges would give the stock a boost, but this is a single event from "last year" and is unlikely to account for a sustained 80% increase over five years. It doesn't explain the drop in retail customers.
(B) A free home loan program would aim to increase retail customers, which contradicts the facts given in the premise. This deepens the paradox rather than solving it.
(C) This option resolves the paradox perfectly. It explains that the drop in retail customers was a deliberate strategic shift, not a sign of failure. It also explains the rise in share price by stating that the new focus—commercial customers—is far more profitable, generating over 75% of revenues. The bank became more valuable by focusing on fewer, but higher-value, customers.
(D) Increased competition explains why retail accounts might drop, but it provides no reason why the bank's share price would rise. In fact, losing customers to competition should logically lead to a lower share price. This only explains one half of the paradox
(E) Being a good paymaster relates to expenses, not revenues. High salary costs could potentially reduce profits and negatively impact the share price. This is irrelevant to resolving the paradox.
Step 3: Determine the best explanation.
Option (C) is the only one that logically connects and explains both parts of the paradox: the decline in one metric (retail accounts) and the rise in another (share price) by introducing a third, overriding factor (a strategic shift to more profitable commercial clients).
Therefore: The bank switching its customer base to more profitable commercial clients best accounts for the observed movements. \[
\boxed{\text{(C)}}
\]