For Achieving this one, I have Tried with vf page 
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<apex:page controller="RetrieveAllObjOnDrpDwn"  showHeader="false" sidebar="false">
    <apex:slds />
    <apex:form id="form">
        <apex:pageBlock title="Choose One Object">
        <apex:selectList value="{!selectedObject}" size="1">
            <apex:selectOptions value="{!objectOptions}" />
            <apex:actionSupport reRender="form" event="onchange" />
        </apex:selectList>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Apex Controller
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public class RetrieveAllObjOnDrpDwn {
    public String selectedObject { get; set; }   
    public SelectOption[] getObjectOptions() {
        SelectOption[] results = new SelectOption[] { new SelectOption('','-- None --') };
        for(SObjectType sType: Schema.getGlobalDescribe().values()) {
            DescribeSObjectResult res = sType.getDescribe();
            results.add(new SelectOption(res.getName(), res.getLabel()));
        }
        return results;
    }
}

Pls Let me know, going forward steps

 

hello,
i have two standard object Account(parent) and Contact(child) .so i wanna create a custom field  'sequence number' on Contact .
suppose my Account(parent) Object have 5 Contact(child) the 'sequence number' record is 1,2,3,4,5      Than i will delete the One 'sequence number' record 3  so my 4 number come to 3  and 5 number is come to 4.    the new 'sequence number' is 1,2,3,4  
if i will delete any record than the sequence of record number is maintan.
this is done help by 'Apex Trigger'.
help my and example with code