a) When the magnification is positive, the nature of image is virtual and erect. In this, case, the magnification is positive, so the nature of image is virtual and erect.
b) The value of magnification given here is 0.75(which is less than 1), so, the image is smaller than the object or diminshed. A virtual, erect and diminished image can be formed only by a concave lens, so the nature of lens is concave.