A Transition metal is different from a non-transition metal because

Explanation

Transition metals are a group of metals that have similar properties due to their partially filled d-orbitals. These metals have unique characteristics that distinguish them from non-transition metals. The question is asking about one of the differences between transition and non-transition metals.

Option A suggests that a transition metal has an octet configuration. However, this is not true. Transition metals usually have more than eight electrons in their outer shell, and they can also have incomplete d-orbitals.

Option B states that a transition metal is very stable. While transition metals are generally stable, this is not a defining characteristic that sets them apart from non-transition metals.

Option C mentions that a transition metal is coloured. This is true for many transition metals, but not all of them. Also, some non-transition metals can be coloured too, so this is not a definitive characteristic.

Option D is the correct answer. A transition metal has incomplete outer shell d-electrons. This is what sets them apart from non-transition metals. The d-electrons in transition metals are responsible for their unique physical and chemical properties.

Option E states that a transition metal has no electron in the d-orbital. This is not true, as transition metals have electrons in their d-orbitals.

In summary, the correct difference between a transition metal and a non-transition metal is that a transition metal has incomplete outer shell d-electrons. This is what gives them their unique properties.