So (a) is a proposition. It doesn't matter whether or not fish can swim; the fact is that this statement may be true or false.
(b) is a proposition. We don't know whether or not it is true, but the fact is that either you will be there in three days time, or you won't.
(c) is not a proposition. It's not making a true-or-false statement.
(d) is a proposition. It's either 3.30 pm or it isn't.
(e) is not a proposition. Don't confuse this with the statement 'You are married' which is either true or false.
(f) is either true or false. So it is a proposition.