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How can I extract a single band from multi-band raster in QGIS?

I have an remote sensed image which has 6 bands (including NDVI band), I want to display each band separately, but have no idea how to do. I have seen some questions similar here but none worked for me.

The original image (has 6 bands) is:

I want to display the band 6 which should be like this:

But I tried gdal_translate, and couldn't get the correct result.

What I have got is:

This is a display issue: you want to display a continuous band using categories. You do not need to split your image to create a new new image: this can be done directly on the multiple band image, and you can add the multiple band layer multiple times on the map.

Go to layer properties > Symbology

Select singleband pseudocolor

Choose the band that you want to display

Select a color ramp

Select an interpolation method (discrete is OK)

Select a mode (I suggest quantile)

Select a number of classes

If you want to change some colors, double clic on the color. And if you want to change a threshol value, double clic on the threshold value.

As a remark, your band 6 doesn't look like the NDVI that you could derive from your image. It is more like some interpolated soil properties (or smoothed NDVI, but if you have a NIR band you could have a more precise one.)

EDIT: I now see from one of your comments that you don't use QGIS 3. In QGIS 2, this would be similar except that you must select "style" in the layer properties.

You can use Rearrange bands tool from processing toolbox but select one band at a time and save the output as separate file:

Now the selected is only one band, and save the image as a new file:

The tool exists in Processing toolbox -> GDAL -> Raster Conversion -> Rearrange bands in QGIS 3.4.5 for the above tool.

Input 6 bands:

Output 1 band:

Repeat to select the bands you want or Run in Batch mode at the bottom of the tool.

NDVI image with 6 bands?? It's not clear what you are asking. NDVI should be a single band raster, with values between -1.0 to 1.0. You might have a colored image with different values of NDVI, but it would then be 3 bands (RGB) not 6. And separating out one of the colors would not be useful in my opinion...

If you are trying to separate out the NDVI values above a certain threshold, then you could use the QGIS raster calculator with an expression like:
("ndvi" > 0.25) * "ndvi"

See here for the relevant QGIS documentation