Step 1: Verify the assertion.
Transition metals (with some exceptions like Mn) are known for remarkably high melting and boiling points compared to neighbouring s-block and p-block metals, so the assertion that transition metals have high melting points is true.
Step 2: Verify the reason.
Transition metals possess incompletely filled d-orbitals in the ground state (e.g., Fe is [Ar]$3d^6 4s^2$, Cr is [Ar]$3d^5 4s^1$). This is factually true for most d-block elements.
Step 3: Check if the reason explains the assertion.
Incomplete d-orbitals provide unpaired electrons that participate in strong covalent-character metallic bonding in addition to the usual electron sea. The more unpaired d-electrons, the stronger the interatomic bonding and the more energy required to melt the metal.
Step 4: Conclusion.
Both the assertion and the reason are true, and the incomplete d-orbitals directly cause stronger metallic bonding, which explains the high melting points.
\[ \boxed{\text{Both A and R are true; R is the correct explanation of A}} \]