Transition elements are:

In the $3d$ series of the $d$-block elements,which element has the maximum atomic size?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: The first ionization enthalpy of $3d$ series elements is more than that of group $2$ metals.
Reason $(R)$: In $3d$ series of elements,successive filling of $d$-orbitals takes place.
In the light of the above statements,choose the correct answer from the options given below:

The $CORRECT$ ionisation energy order is

Reactivity of transition elements decreases almost regularly from $Sc$ to $Cu$. Explain.

Which of the following is the heaviest metal?