If you are talking about art in general, you need to say 'Art is interesting.' You would only say 'The art..' if you are talking about the artwork in a particular exhibition, for example. You might say this: 'How was your visit to the gallery?' 'Well, the art was interesting, but the gallery was so crowded that I could hardly see anything properly.''

"The art is interesting." is correct. "Interesting" is the present participle of "interest" to form the continuous present. "Interest" alone is a noun, "The interest".

The art is interesting