What is the relationship between ka and kb with kw?
the sum of ka and kb equals the auto-dissociation constant for water. the product of ka and kb equals the auto-dissociation constant for water. the quotient of ka and kb equals the auto-dissociation constant for water. the difference of ka and kb equals the auto-dissociation constant for water.

Answer is: The product of Ka and Kb equals the auto-dissociation constant for water.

Reaction of  auto-dissociation for water: H₂O ⇄ H⁺ + OH⁻.

Water dissociates (autoionization) to form hydrogen ions (H⁺) and hydroxide (OH⁻) ions. The protons (H⁺) hydrate as hydroxonium ions( H₃O⁺).

The Kw (the ionic product for water) at 25°C is 1·10⁻¹⁴ mol²/dm⁶ or 1·10⁻¹⁴ M². Concentration of hydrogen ions and hydroxide ions in pure water are the same.

Ka · Kb = Kw;  the ionic product of water.

it is b that is the answer.