Final answer:
The sentence is corrected by inserting a semicolon between two independent clauses: 'I decided to climb the seven flights of stairs; Mother took the elevator.' This punctuation mark is appropriate as the two clauses are closely related and not joined by a conjunction.
Step-by-step explanation:
To correct the sentence by adding the appropriate punctuation without changing the wording, one might write:
I decided to climb the seven flights of stairs; Mother took the elevator.
This correction involves the insertion of a semicolon to separate two independent clauses that are closely related. The semicolon is correct here because it joins two independent statements that are not connected by a conjunction such as 'and' or 'but'.