Best alternative to Nikon 14-24, 24-70 and 70-200mm?

Alternatives to 14-24mm lens:

- Sigma 10-20mm f/4-5/6

- Tokina 12-24mm f/4

- Tokina 11-16mm f/2.8

- AFS Nikkor 10-24mm f/3.5-4.5G

Alternatives to 24-70mm lens:

- Tokina 16-50mm f/2.8

- Tamron 17-50mm f/2.8

- Nikkor 17-55mm f/2.8

- Nikkor 16-85mm f/3.5-5.6 (if you're willing to settle for smaller f/stop)

I would skip 24 or 28-xx standard zoom lenses because the wide-angle isn't nearly wide enough on DX cameras like the D90

Alternatives to 70-200mm lens:

- AFS Nikkor 70-300mm f/4.5-5.6

- AF Nikkor 80-200mm f/2.8D

- Sigma 70-200mm f/2.8

- Tamron 70-200mm f/2.8

Nikon 14 24 Alternative

You really have to sit down and figure out what exactly YOU need. I would keep both the 24-70 mm and 70-200 mm lenses. They will cover almost 80% of the subjects you may be shooting. Add the 12-24 mm and you will be shooting almost anything there is in your interest sphere. If you are more into macro than landscape, then substitute a 60 mm f/2.8 macro for the 12-24 mm lens

I don't understand what alternative means in this case - other brands, other sizes, what?

Other brands there aren't any as good. Other sizes they're plenty just do some research.