The question asks about the strength of the conjugate acids of the bases provided. I have attached an image that provides the actual list of bases along with their Kb values. The Kb value tells us the relative strength of the base according to the general reaction:
B + H⁺ → BH⁺
Kb = [BH⁺]/[B][H⁺]
Therefore, the stronger the base, the greater the concentration of BH⁺ and the greater the value of Kb. In the reaction shown above, BH⁺ is the conjugate acid to the base, B. Therefore, a strong base, B, will produce a weak conjugate acid, BH⁺. So a large value of Kb for the base means that it forms the weakest conjugate acid.
Of the bases provided, NH₂CH₃ has the largest value of Kb at 4.4 x 10⁻⁴. This tells us that NH₂CH₃ is the strongest base of the group and will produce the weakest conjugate acid which will be NH₃CH₃⁺.
