The correct answer of this question is : Negative focal length and virtual images.
EXPLANATION:
The concave lens is that transparent spherically refracting surface which is diverging in nature. It is so because it is bulged at the edges and thinner in the centre.
The focal length of the concave lens is always taken negative.
As the lens is diverging in nature, it always forms virtual,erect and diminished image. It means the image formed by concave lens has less size as compared to object.
Hence, the correct answer of this question is that concave lenses have negative focal length and always form virtual images.
