Step 1: Look at the Picture:
Just observe the lines in the picture. They are dark, wide and even, and they enclose each patch of colour.
Step 2: Test option 1.
Pencil lines are faint and grey. The picture has no such lines, so reject option 1.
Step 3: Test option 2.
Thin lines are narrow. The lines here are wide, so reject option 2.
Step 4: Test option 3.
Wide black lines enclosing the colours are what we see, so option 3 fits.
Step 5: Test option 4.
Metal is a material and not an outline type, so reject option 4.
Final Answer:
The painting uses thick black outlines, option 3.
\[\boxed{3}\]