Image formed by a convex spherical mirror is
Virtual
Explanation for the correct option(s)-
The image formed by a convex mirror is always virtual, erect, and diminished.
The explanation for the incorrect option(s)-
Option (b)
A convex mirror does not form a real image.
Option (c)
A convex mirror always forms a diminished image.
Option (d)
A convex mirror always forms an erected image.
Hence, option (a) is the correct answer.