Step 1: Look at the Picture:
Ask what the surface of the picture tells you. The colours are solid, dull and matt, with no see-through washes and no oily shine.
Step 2: Test option 1.
Oil needs a canvas and gives a shiny look. The picture does not have that look, so reject it.
Step 3: Test option 2.
Opaque matt colour on paper is the gouache method, and it matches the flat tones seen. Keep this option.
Step 4: Test option 3.
Silk with watercolor would show light, thin, transparent colour. Not seen here, so reject it.
Step 5: Test option 4.
Acrylic on board is a modern plastic based colour and not linked with this folk inspired work, so reject it.
Final Answer:
Flat opaque matt colour points to gouache on paper, which is option 2.
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