Which of the following describes Kerkhoff's principle $($which dates to the $19$th century$)?$

Question $7$ options:

a$)$

no algorithm is unbreakable

b$)$

the security of a cipher depends on the security of the key, not the security of the algorithm

c$)$

all algorithms are secret

d$)$

the security of a cipher depends on the security of the algorithm, not the security of the key