Halftoning is defined as

Concept: A display device can produce only a fixed set of intensity levels at each pixel — a bilevel device produces just two, dot on or dot off. Halftoning (halftone approximation) trades spatial resolution for intensity resolution: neighbouring pixels are grouped into a small cell, and the number of dots switched on inside that cell is varied so the eye, which averages over the cell, perceives shades the hardware itself cannot generate.

Applying it here: A definition of the technique must carry both halves of that trade — the device supplies only the fewest (minimum) intensity levels, and the purpose is to obtain increased visual resolution of intensity. The statement "a technique for using minimum number of intensity levels to obtain increased visual resolution" states exactly that pairing.

The mechanism, step by step:

1. Take a bilevel display: each pixel is either on or off, so the hardware supplies 2 intensity levels.

2. Group the pixels into a 2 × 2 cell. The cell can carry 0, 1, 2, 3 or 4 switched-on dots.

3. The eye averages over the cell, so 5 = 22 + 1 apparent intensity levels are seen from a 2-level device.

4. Generalising, an n × n cell on a bilevel device yields n2 + 1 apparent levels, at the cost of cutting the effective spatial resolution by a factor of n in each direction — resolution traded for shading.

Contrasting the other statements by value:

• "multiple intensity levels" — states only that more than one level exists and leaves the count open, so it does not express the intensity-limited hardware condition the technique is defined against.

• "maximum number of intensity levels" — describes hardware already used at its full intensity depth; with the full range available each shade is rendered directly at one pixel and no approximation is needed.

• "appropriate number of intensity levels" — names a selection criterion rather than the scarcity of levels, so it does not identify what the technique actually does.

Cross-check: Printed media use the same trade. A newspaper photograph is printed with a single ink density — ink or no ink, two levels — and varies the number and size of dots inside each cell so a full grey scale appears to the reader. Minimum intensity levels, increased apparent shading: the same definition, in print.