Explanation

(a)
In octahedral structure $MX_{6}$, the six hybrid orbitals $(sp^{3}\ d^{2})$ are directed towards the corners of a regular octahedral with an angle of $90{^\circ}$. According to following structure of $MX_{6}$ the number of $X - M - X$ bonds at $180{^\circ}$ must be three.
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