inverted structure with the use of 'comma'

[Unregistered-jack]

Hi....

I wonder which of the sentences below are acceptable (grammatically or else).

a) Under no circumstances is robbery allowed. (inverted structure:AdvP-V-S)
b) Under no circumstances robbery is allowed. (not inverted:AdvP-S-V)
c) Under no circumstances, is robbery allowed. (inverted with comma)
d) Under no circumstances, robbery is allowed. (not inverted with comma)

I think only the sentences a) and d) are acceptable in terms of (strict)grammaticality.
Sentence b) seems somewhat vague, if it could be used in speech. This is not important.
However, sentence c) is NOT acceptable ....because comma is used in an inverted structure.

As far as I know, commas shouldn't be used in (S-V) inverted structures.
Am I right? Please help.

[Casiopea]

Symbols
grammatical
semantically awkward
ungrammatical

a) Under no circumstances is robbery allowed.
b) Under no circumstances robbery is allowed.
c) Under no circumstances, is robbery allowed.
d) Under no circumstances, robbery is allowed.

As far as I know, commas shouldn't be used in (S-V) inverted structures. Am I right?

That's correct.