Log switch is
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It is the point at which ORACLE starts writing to one online redo log file and begins writing to another at same time
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It is the point at which ORACLE ends writing to one online redo log file and begins writing to another
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It is the point at which ORACLE starts writing to one online redo log file and ends writing to another
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It is the point at which ORACLE ends writing to one online redo log file and starts writing to another
A log switch occurs when Oracle finishes writing to one online redo log file and begins writing to the next one. This happens automatically when the current redo log fills up. Option B correctly describes this as ending one file and starting another. Options A and C incorrectly state Oracle starts writing to both files simultaneously, which is not how log switching works. Option D is nearly correct but has awkward phrasing with 'starts writing to another' being less precise than 'begins writing to another'.
A log switch is the point at which Oracle stops writing to the current online redo log file (having filled it) and begins writing to the next one in the cycle. The correct option captures this 'ends...begins another' sequence. The other options either describe simultaneous writing to two logs (incorrect — Oracle writes to only one online redo log group at a time) or reverse the start/end order incorrectly. Log switches trigger checkpoints and, in ARCHIVELOG mode, archiving of the just-filled redo log.