Answer:
3 electron pairs
Let's take BF3 for an example.
When you find the valence electrons for this molecule --> [3+(3x6)], you'll get 24e-. You draw the Lewis dot structure and each of the F atom would have six dots around it (a total of 18 because there's 3 F atoms) and the B would have three lines [one line attached to each F. One line would equal to 2 valence electrons] and you get a total of 24 valence e-.
The question asks how many groups are on the CENTRAL atom, meaning on the B. It would be three
