What XPATH expression to use

[mje]

You're close, but your xml is invalid to begin with.

<Data>
  <Objects>
    <A ExternalId="A"/>
    <B ExternalId="B"/>
    <A ExternalId="C"/>
    <B ExternalId="D"/>
  </Objects>
</Data>

A valid XPath would be "/Data/Objects/A[@ExternalId='C']"