What is the best solution to provide employees, customers, and partners access to all the information they need to resolve their cases through Salesforce communities?

Answer options:

A.Articles
B.Ideas
C.Answers
D.Knowledge Base

Correct Answer: D 
Salesforce Knowledge quickly offers articles to your customers and agents in a community. Knowledge articles provide accurate information to customers whenever they need it. 
Option A is incorrect because a channel refers to the medium by which an article is available. Salesforce Knowledge offers four channels where you can make articles available. 
Option B is incorrect because Ideas community provides an online, transparent way for you to attract, manage, and showcase innovation. 
Option C is incorrect because answers are no longer supported in all Salesforce Orgs as of the spring’18 release. 
Reference: 

https://help.salesforce.com/articleView?id=networks_knowledge_access.htm&type=5