How to pass the id from visualforce page when i click the preview button

Try below page code.

<apex:page standardController="FeedbackMain__c" extensions="FeedbackMainlistController">
<apex:form >
<apex:pageBlock id="pg">
<apex:pageMessages />
<apex:pageBlockTable value="{!feed}" var="a" rowClasses="oddrow,evenrow,dataTableRow" styleClass="fontfamily" id="Details1" style="width:50%;">
<apex:column headerValue="Feedback Number" style="width:10%;">
 <apex:outputField value="{!a.Name}" id="test" />
<apex:variable var="vara" value="{!a.Name}" />
</apex:column>
<apex:column headerValue="Feedback Preview" style="width:45%;" >
  <apex:commandButton value="Preview" style="width:35%;" >
  <apex:actionSupport action="{!queslist}" event="onclick" reRender="pb">
    <apex:param assignTo="{!selectedId}" value="{!a.id}" name="selectedId"/>
  </apex:actionsupport>
 </apex:commandButton>
</apex:column>
</apex:pageBlockTable>
</apex:pageBlock>
</apex:form>
</apex:page>