Which of the following will not be a good fuse design ? a; low-melt wire encased in glass . b; low-melt wire encased in ceramic. c; low-melt wire encased in iron . d- low-melt wire encased in a tube of air

Low-melt wire encased in iron doesn't sound like a good idea.
I can think of two things wrong with it:

1).  A fuse is designed to melt if the wire gets too hot.
If the wire is mounted in iron, the heat from the wire will
just soak away into the iron, and the fuse-wire will never
get hot.  So this fuse would not do its job and protect you
or your circuit.

2).  Iron is a pretty good conductor of electricity too.
So if the fuse wire is mounted in iron, then touching the
iron is almost the same thing as touching the fuse-wire.
This design would be very dangerous, and you'd have to
encase the iron in something ELSE ... like maybe glass
or ceramic ... to prevent people from touching the iron.