Which of the following equations gives a circle?

Question

\[\begin {array} {1 1} (a)\;r= 2 \sin \theta & \quad (b)\;r^2\cos 2 \theta=1 \\ (c)\;r(4 \cos \theta+ 5 \sin \theta)=3 & \quad (d)\;5= r (1+\sqrt 2 \cos \theta) \end {array}\]