Who among the following observed that ‘The artistic critic, like the mystic, is an antimuonium always”?

The correct option is Oscar Wilde.

This observation is made by Oscar Wilde in his famous essay "The Critic as Artist" (first published in 1890 in The Nineteenth Century and later included in his 1891 collection of essays titled Intentions).
Note that the word "antimuonium" in the question is a typographical variation of "antinomian". Wilde's original text reads: "The artistic critic, like the mystic, is an antinomian always."

An antinomian is someone who rejects established moral laws, social conventions, or orthodox rules in favor of personal faith, individual conscience, or aesthetic sensibility.
In the essay, Wilde argues that the true critic is not a passive judge of art who follows standard rules, but a creative artist in their own right who seeks subjective, individualistic experiences of beauty and art, transcending conventional societal expectations.