i draw multiple object in ink scape but i want only one object Export is it possible or not?

how many extension support to Export a ink scape file like plt Dxf SVG?

You can use File menu > Export Bitmap, with the Selection option, to create a PNG file of only the object that's selected. Or you could use Edit menu > Make a Bitmap Copy to do essentially the same thing.

To get a PLT or DXF file, you need to use Save As. Be sure to type the extension after the file name, AND select either PLT or DXF from the dropdown. Sometimes the file doesn't get saved if you don't do both

Unfortunately, you can't use Save As, to save only the selection. You have to save the whole document. However, there may be a "workaround" which will work for you. For example, you could put that object in a new layer, and hide all the other layers, before saving. Or you could use File menu > Save a Copy, which will save an identical SVG. Then you could open the new SVG and delete everything except the object, and then use Save As.

Or maybe someone else will have a clever trick?

I am wondering with one thing. How can a bitmap image can be compared with PNG image. To create bitmap image it is have to save .bmp extention and for PNG it have to save as .png extension. It is enough at all.

Images with the BMP extension are usually uncompressed and very large. PNGs are usually quite small. They look essentially the same.

However, Inkscape cannot Save As or Export BMP. Where Inkscape uses the terminology "Export Bitmap" it really means "export raster formats". In the future, Inkscape will be able to export other raster formats, perhaps including BMP (I'm not sure). But for now, only PNG.