regular grammar

Question

If a is a terminal and S, A, B are three non-terminals, then which of the following are regular grammars?

(a) S → ε, A → aS|b
(b) A → aB|a, B → bA|b
(c) A → Ba|Bab
(d) A → abB|aB
 

answer given is b. but I think all are regular grammars. please clear my doubt.

Comments

S--> epsilon is fine ,but A-->aS|b is not valid,so A is false.

Had the option been S--->Epsilon its right.

Option is false because some part of the option given is invalid

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A-->aS|b is not valid ? why .

We can subsitute S with epsilon. Is'nt??

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you can never reach it to A that's why its invalid.