If there were only three electron groups around an atom how would they be arranged

They would be arranged in a trigonal planar orientation.

If there were only three electron groups around an atom they tend to repel each other and gain a trigonal planar orientation. A molecule with three electron groups orients the three groups as far apart as possible. They adopt the positions of an equilateral triangle 120° apart and in a plane. The shape of such molecules is trigonal planar. An example is BF3:

trigonal planar orientation.

Explanation:

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a is the correct answer