[Solved]-How to underline text within a node's label in d3 force based layouts?-d3.js

As Matt Walters said, you can change the text colour and add underlining with CSS, including adding hover effects.

However, I would highly recommend that you implement your links as actual <a> element hyperlinks, which are perfectly acceptable in SVG. I've tested it, and you can either wrap the text element in a link, or put the link inside the text element, but surrounding the actual text content. The only complication is that you have to use xlink:href for the url attribute, instead of simply href.

Here's a force-directed graph I created for someone else's debugging, adapted to turn all the text labels into links:
http://codepen.io/AmeliaBR/pen/AoFHg

Key code in the update function:

hypertext  = hypertext.data(force.nodes());
// hypertext refers to a selection of text elements, joined to the data

// Add labels for new nodes:
hypertext.enter().append("text") //add the text element
         //add any unchanging attributes of the <text>:
         .attr("text-anchor", "middle") 
    //add a link element within each text element
    .append("a") 
         //add unchanging attributes of the <a> link
         .attr("target", "_blank") //set the link to open in an unnamed tab
         .attr("xlink:show", "new"); 
          //show="new" is an XML way to tell the link to open in a new tab/window.
          //Either attribute *should* work on its own, but best use both to be safe.

// At this point, each new <text> element contains an <a> element, but the
// variable hypertext still refers to the text elements.

// Remove any outgoing/old text elements for non-existent nodes:
hypertext.exit().remove();

// Compute data-based attributes for entering and updating texts:
hypertext.attr("x", 8) //attributes of the <text> elements
      .attr("y", "0.3em")
  //Select the existing <a> element within each <text>
  .select("a")
      //Update the link attributes.  Note that the <a> element was never given
      //its own data, so it inherits the data from the parent <text> element.
      .attr("xlink:href", function (d) {
          return "http://example.com/" + d.name; //create url from data
      })
      //Set the text within the link, which in this case is the only text
      //within the text element.
      .text(function (d) {
          return d.name; //link text content
      });

Using the link element created all the necessary functionality, but it didn't add the default HTML link styles. For that, there's CSS:

text a {
  fill: navy;
}
text a:visited {
  fill:darkpurple;
}
text a:hover, text a:active {
  text-decoration: underline;
  fill:darkred;
}