Atrait controlled by a dominant gene will be expressed even when the corresponding gene in the other half of the pair is different will be expressed only if it is paired with two recessive genes will not be expressed if it is paired with another dominant gene will not be expressed when the corresponding gene in the other half of the pair is different

I'm assuming that the questions and choices are set up like this:

A trait controlled by a dominant gene . 
a.) will be expressed even when the corresponding gene in the other half of the pair is different 
b.) will be expressed only if it is paired with two recessive genes
c.) will not be expressed if it is paired with another dominant gene
d.) will not be expressed when the corresponding gene in the other half of the pair is different

If this is the case, then the answer is a.) 

Traits controlled by dominant genes will be expressed even when the corresponding gene in the other half of the pair is different

Hope this helps! :)