Heavy water and ________ is one of the substances used as moderator in reactors.

The correct option is Graphite.

In nuclear reactors, a moderator is a medium used to slow down fast neutrons produced during nuclear fission. Slowing down these neutrons is crucial because slow-moving (thermal) neutrons are much more likely to cause further fission in fissile isotopes like Uranium-235, thereby sustaining the nuclear chain reaction.

To be an effective moderator, a substance must have specific properties:
1. A low mass number (light nuclei), so that neutrons transfer maximum kinetic energy during collisions.
2. A very low probability of absorbing neutrons (low neutron absorption cross-section) to prevent stopping the chain reaction.

Among the substances commonly used as moderators in nuclear reactors, the most prominent are:
• Heavy water (deuterium oxide, $D_{2}O$)
• Light water (ordinary water, $H_{2}O$)
• Graphite (a crystalline form of pure carbon)

Since the question asks for a substance used alongside heavy water as a moderator, Graphite is the correct fit among the choices provided. Other options like Sea water (which contains impurities that absorb neutrons), Titanium, and Cobalt are not used as moderators because they do not have the necessary physical properties to slow down neutrons efficiently without absorbing them.