`a)`

`sin \alpha=(\sqrt{2})/2`

`=>\alpha=sin^(-1)((\sqrt{2})/2)=45^o`

`b)`

`cos \alpha=(\sqrt{3})/2`

`=>\alpha=cos^(-1)((\sqrt{3})/2)=30^o`

`c)`

`cot \alpha=3/4`

`=> tan \alpha=4/3`

`=>\alpha=tan^(-1)(4/3)≈53^o`