Lattice Primitive Unit Cell

is geometric structure that when translated by the primitive vectors will fill up the entire space of the lattice. The Lattice Primitive unit cell will contain only one lattice point.

Any set of two (linear independent) vectors which contain a single lattice site can define the primitive cell of a crystal.

The pair of vectors labeled 5 and 6 do not define a primitive cell. Vectors 5 are not primitive because they enclose two full lattice sites and vectors 6 are not valid because they are not linearly independent and thus do not define a 2-D expanse.