Several of the options generate especially common or interesting rules. For example, a, b, c, d generate shifts along the four diagonal directions; the new cell simply copies one of its neighbors.
An ``eater'' type of rule is one in which a cell acquires the state of whichever of its neighbors is poised to ``eat'' it; the food chain is generally cyclic, so that 1 eats 0, 2 eats 1, and so on until 0 eats 3. Lacking a central cell, there are four candidates as to who will be the meal, consequently four rules, A, B, C, D.