In what kind of art can mass be observed?

If you're asking what kind of art can be viewed by the
masses, then you have to pick a time period.  Prior to the Modern, or industrial age,
art for the masses was typically limited to themes of religion and art which supported a
State.  Each culture had its own art for the masses and then there was high art, often
associated with elitists or aristocrats or clergy.  It depends on the time periods and
in which parts of the world you're talking about with regards to things like: the degree
of rigid definition between "high" art and mass art.  Clearly, the lines of restriction,
censorship and availability of art, especially based on class systems will vary
depending no time and place.  But in general, art has been practiced by all classes;
from cave paintings to the Sistine Chapel; each to represent or suppose some idea,
culture, reality or metaphysical concept.  As Modernism developed, along with the age of
mechanical reproduction of art (photograph, video, movie, etc.) art of all types from
all periods of history became available to the masses.  This movement in art continued
the blurring of lines between concepts of high art and pop (or kitsch) art.  Anything
from cave painting to Monet can be seen in advertising on the internet set to Mozart or
Rage Against the Machine.  Everything now overlaps (pastiche).  A more literal answer to
your question is that art seen by the masses would be in public; museums that are free
or art outside where it can't possibly be censored: from churches to installations to
graffiti.