Which of the following has highest solubility in water?
You can't predict solubility reasoning, you have to remember solubility rules in your case this one:


  
  Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and $\ce{Al^3+}$ are insoluble. Thus, $\ce{Fe(OH)3, Al(OH)3, Co(OH)2}$ are not soluble. 
  


So in this case $\ce{Ca(OH)2}$ even if slightly soluble theoretically should be the right answer.