Is there is any way we can converted select picture only in to grayscale in adobe illustrator?

Yes you can. An image placed on illustrator's artboard whether in cmyk or rgb mode can be both converted to gray scale, provided you embed those images after you placed them on the artboard. How will you do that?

After placing an image or picture on the artboard, click embed (see image below).

And from "Edit Menu", scroll down to Edit Colors, Convert to Grayscale.

On artwork or object, drawing created inside within illustrator's artboard, there's no need to embed, select the object or multiple objects drawing, just go to edit colors, convert to grayscale, that's it.

Haziq,

If you wish to retain the original coloured object/image, remember to keep the original as backup and work with a copy (usually recommended before destroying artwork).

Or there is a silly way to work reversibly with the original whether vector object or raster image, and the latter can be linked to keep the file size down.

And any effect way, as the one suggested by Ton, is always reversible.

Or you can apply Effect > Rasterize and choose greyscale.

The advantage is that it is an effect and can be changed or turned off.

The disadvantage is that your vectors will behave like an image.