(a) No, the linear speed of a comet is not constant.
(b) No, the angular speed of the comet is different throughout the orbit.
(c) Yes, Angular momentum is constant.
(d) No, speed is not constant.
(e) No, potential energy is not constant.
(f) Yes, Angular momentum and total energy at all points of the orbit of a comet moving in a highly elliptical orbit around the Sun are constant. Its linear speed, angular speed, kinetic, and potential energy varies from point to point in the orbit.