[Note: When the eye is separated from the lens, the angles
subtended at the eye by the first object and its image are not
equal ]
(c)
First, grinding lens of very small focal length is not easy More
important, if you decrease focal length, aberrations (both spherical
and chromatic) become more pronounced So, in practice, you
cannot  get a magnifying power of more than 3 or so with a simple
convex lens