Option B is the correct answer.
We are to choose the adverb which indicates the suitable time period beginning from which they will never try trekking in the rain again.
'Herewith' means along with.
'Hereby' means as a result of.
'Herein' means inside.
None of the options A, B or D indicate a time period.
Option B 'Hereafter' means 'in future time or after this time period' .
Hence Option B is the suitable option.