In a flow you can directly reference a decision rule in both the decision shape and the connector. In answering your questions, here are my responses.
How will we call a Desicion from Connector?
You use the connector to call the When rule:
1. We can call Decision from Decision shape -
you are correct, you can call a decision tree or decision table to name a few
2. We can call When rules from FORK -
unfortunately this is incorrect. The fork shape acts as a placeholder and there is no way to configure the fork to call a decision rule.
3. We can Call Decision rules from a utility by calling activity and decision methods
the utility shape references activity rules and while you can reference decision rules by using methods within the activty you can not reference the decision rules directly from the utility flow shape.
How will we call a Decision from Connector??
change the connector type to When and then reference the when rule. By changing the liklihood you will also affect the order in which the when rules are evaluated.