To excel in modern competitive sports requires not only rigorous physical training but also to develop mental fortitude, as much as athletes need to analyze their opponents' strategies.
Correct: D
The sentence uses the correlative conjunction 'not only X but also Y,' which requires X and Y to be grammatically parallel. The first part, 'rigorous physical training,' is a noun phrase. Therefore, the second part should also be a parallel noun phrase or gerund phrase. The original sentence's 'to develop mental fortitude' (an infinitive phrase) breaks this parallelism. Furthermore, the ending 'as much as athletes need to analyze their opponents' strategies' is awkward and verbose. Option D correctly uses parallel gerund phrases ('developing mental fortitude and analyzing their opponents' strategies') to align with 'rigorous physical training,' making the entire list of requirements parallel and concise. Option A and E retain the infinitive, breaking parallelism. Option B uses 'the development of mental fortitude' (a noun phrase) but retains the awkward 'as much as athletes need to analyze.' Option C uses a gerund but then switches to 'an analysis,' breaking parallelism within the 'as well as' clause.