Which of the following statements is true?

a$)$ A convex lens has $5$ diopter power having a focal length $0\mathrm{.}20m$

b$)$ A convex lens has $5$ diopter power having a focal length $-0\mathrm{.}20m$

c$)$ A concave lens has $4$ diopter power having a focal length $5m$

d$)$ A concave lens has $4$ diopter power having a focal length $-5m$

Select one:

a$.$ b

b$.$ c

c$.$ d

d$.$ a