a is awkward, but people might say it speaking casually.
b is reasonable but a little odd because this phrasing implies this is a regular, recurring worry.
c is awkward again.
d is fine.
e is fine.
f feels too wordy.
e is probably most natural.
'whether' can be replaced with 'whether or not' in any of them.
'wonder' might be the better verb for a general concern as these sentences seem to express.
'I'm worried that long skirts might not still be in fashion' is the sort of phrase that might be used for an immediate worry, like when trying to decide what to wear to a particular event.