Suppose the adjacency relation of vertices in a graph is represented in a…

The query that cannot be expressed by a relational algebra expression of constant length is the one involving reachability from a given vertex. This is because reachability requires computing the transitive closure of the adjacency relation, which involves paths of arbitrary length. Relational algebra does not support recursion or iteration, so it cannot express queries that depend on unbounded path lengths in constant length. The other options—listing adjacent vertices, self-loops, and cycles of less than three vertices—can all be expressed using finite joins and selections, which are expressible in constant-length relational algebra.