The correct order of the first ionization enthalpy is:

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Strategy:
 Ionization enthalpy generally decreases down a group due to increasing atomic size. However, anomalies can occur due to poor shielding effects (like -block contraction in Group 13).

Step 1: Compare Group 13 elements
The elements are Boron (B), Aluminum (Al), and Gallium (Ga).
- Boron: Smallest size, highest   Highest IE.
- Aluminum vs Gallium: Gallium () has  electrons. The poor shielding of the nucleus by these  electrons results in a smaller atomic size and a slightly higher IE tha

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