on my tenor sax, I have trouble getting the D (4th line). All the notes below the D are sound lovely, then when I play the D it goes very very rusty-sounding and not clean at all. As soon as I play a note about the D, it sounds fine again. I press all three keys with my left hand, and the first three with my right hand as well as the octabve key (dunn if that`s the right name for it?) Is this the right fingering? If so, why does my D not play properly!?

my sax teacher has told me that it is just a matter of practice. and yes the octave key is the right name for it.

I`m willing to bet that it`s your 2nd octave key that isn`t opening up all the way. Judging by how unsure you were of how to finger a D, I assume you also don`t know about this 2nd and smaller octave key. Look at your left palm keys, the 3 keys that kinda stick out on the side. Then press the bottom right key, that`s the high F key. Now look at where the key opens up, (this should be the top key on your saxophone unless you have a high F# key, then it would be the second highest note) Right next to the high F key there is a tiny key and pad that are closed when nothing is pressed, but now finger any note that presses your L3 finger and the octave key. (i.e. G, D, etc.) This is your second octave key. Now dont press any keys, this is middle C#, and then finger the D. L1, L2, L3, R1, R2, R3, OK. You should see the second octave key open, and it should open wide enough so that air can easily get out. If it`s not, you`ll get exactly what you`re describing; an unclean and mushy sound.
If that doesn`t help, check your instrument for leaks- you can use a glow stick in the dark (slide it inside the top and look for any light that comes out of the keys when fingering low Bb), or just take it into the shop and have them take a look at it. That problem doesn`t sound like it should cost more than $20.
Hope I could help,
Eric.