Which of the following bicarbonate is insoluble in water?

Bicarbonates of Alkali metals are less soluble than Alkaline earth metals. So compare NaHCO₃ and KHCO₃

Structure of anion in NaHCO₃​ $\left({\mathrm{Na}}^{+}{\mathrm{HCO}}_3^{-}\right)$ and KHCO₃​ $\left({\mathrm{K}}^{+}{\mathrm{HCO}}_3^{-}\right)$ is:

The two oxygen atoms with longer C−O distance are hydrogen bonded in pairs to another CO₃​ group with an O−H...O distance 2.61A, thus linking two CO_3​ groups so as to form a complex anion. [H_2​C_2​O_4​]⁻.This type of approximately coplanar conjugation of two bicarbonate ions is a new one found in acid carbonates and resembles that of a dimeric carboxylic acid. Thus two H bonds dimer type in KHCO₃​(a) and the chain type in NaHCO₃​(b) as shown in figure. 

So due to the presence of two H- bonds dimeric anion KHCO₃​ is more soluble in water as H₂​O being polar and H bonding in dimeric anion of KHCO₃​ makes it more polar.