Answer:
3+
Step-by-step explanation:
Boron belongs to the 3rd group, the Boron family on the periodic table. It has an atomic number of 5. For neutral atoms of Boron, we have 5 electrons in them.
All atoms lose or gain electrons in order to achieve a perfect noble gas configuration which would give them stability. Noble gases are known to be stable and unreactive.
For Boron, it would readily want to lose 3 electrons so as to mimic a Helium, He atom. Helium is the first element in the noble gas series.
When Boron loses 3 electrons, a net charge of 3+ is left on the atom of Boron.