About which other carbon–carbon bonds may rotation occur?

The greatest interaction occurs around the carbon carbon single bond.

Single bonds in organic molecules are free to rotate, due to the 'end-to-end' (sigma) nature of their orbital overlap.

For example, In ethane  rotation about the carbon-carbon sigma bond results in many different possible three-dimensional arrangements of the atoms.  

In carbon carbon double bond and triple bonds the atoms are held together more strongly so they show less rotations as compared to carbon carbon single bond.

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multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.