Which lanthanoid from following has highest ionisation $\left(\mathrm{IE}_1\right)$ enthalpy?

Ionization enthalpy increases across the lanthanoid series due to the increasing nuclear charge combined with the poor shielding effect of 4f electrons.

Neodymium (Nd, Z=60): [Xe] 4f$^4$ 6s$^2$, IE1 ≈ 533.1 kJ/mol
Dysprosium (Dy, Z=66): [Xe] 4f$^{10}$ 6s$^2$, IE1 ≈ 573.0 kJ/mol
Thulium (Tm, Z=69): [Xe] 4f$^{12}$ 6s$^2$, IE1 ≈ 596.7 kJ/mol
Ytterbium (Yb, Z=70): [Xe] 4f$^{14}$ 6s$^2$, IE1 ≈ 603.4 kJ/mol

The magnitude of IE1 increases with atomic number as the effective nuclear charge grows. Ytterbium shows the highest ionization enthalpy due to its position at the series end with a fully filled 4f subshell providing minimal shielding.

The final answer is $\boxed{\text{D}}$