In reported speech, the word order and pronouns will be different from
the original speech, because the speaker has changed. Accordingly, the
tense will also change, in most cases.
Normally, while reporting the speech, the tense will change to one tense back in time.
The verb 'can' changes to its past tense form 'could' in the indirect/reported speech.
Option B:
'Could' is the past tense form of 'can' in indirect speech. Hence
'could not swim' is the correct form and B is right.
Option A: 'Can' must be changed to past tense as the speech is reported. Hence A is incorrect.
Option C: 'She swam' indicates that an action has been performed in the past. The original speech 'can swim without fear' refers to the ability to swim. It does not express an action that has taken place. Hence C is incorrect.
Option
D: The subject pronoun 'I' changes while transforming a direct speech to indirect. This is because the speaker changes.
So, 'I' isn't spoken of from the original speaker's point of view. 'I' has now become 'she', which is in the third person. Hence D is incorrect.