您需要为MailMessage的主体启用HTML，如下所示：

o.IsBodyHtml = true;

也许你应该选择另一个构造函数，使代码更易读。也许像这样：

var mailMessage = new MailMessage();
mailMessage.From = new MailAddress("sender@domain.com", "Customer Service");
mailMessage.To.Add(new MailAddress("someone@domain.com"));
mailMessage.Subject = "A descriptive subject";
mailMessage.IsBodyHtml = true;
mailMessage.Body = "Body containing <strong>HTML</strong>";

完整文档：http://msdn.microsoft.com/en-us/library/System.Net.Mail.MailMessage(v=vs.110).aspx

更新看起来是你的字符串构建给你带来了麻烦。有时候，当把字符串放在一起（或者当它被调用时把它们连接起来）时，正确地得到所有的引号是很棘手的。当创建像电子邮件这样的大字符串时，有一些选项可以让它正确。
First，常规字符串-缺点是难以读取

string body = "Hello, " + name + "\n Your KAUH Account about to activate click the link below to complete the actination process \n <a href=\"http://localhost:49496/Activated.aspx">login</a>";

第二个，逐字字符串-允许在代码中换行，以提高可读性。注意开头的@字符和引号转义序列从\"更改为""。

string body = @"Hello, " + name + "\n Your KAUH Account about to
    activate click the link below to complete the actination process \n 
    <a href=""http://localhost:49496/Activated.aspx"">login</a>"

第三个，字符串生成器。这实际上是在许多方面的首选方式。

var body = new StringBuilder();
body.AppendFormat("Hello, {0}\n", name);
body.AppendLine(@"Your KAUH Account about to activate click 
    the link below to complete the actination process");
body.AppendLine("<a href=\"http://localhost:49496/Activated.aspx\">login</a>");
mailMessage.Body = body.ToString();

字符串构建器文档：http://msdn.microsoft.com/en-us/library/system.text.stringbuilder(v=vs.110).aspx

将消息标记为html o.IsBodyHtml = true

String body = "ur message : <a href='http://www.yoursite.com'></a>"
     o.Body = body;

o.IsBodyHtml = true

您忘记转义“：href=\”.... "〉登录

语法错误：

MailMessage o [...snip...] \n "+<a href=\"http://localh [...snip...]
                              ^--terminates the string
                                ^^^^^^^^^^^^^^--interpreted as code

string url = lblHidOnlineURL.Value + hidEncryptedEmpCode.Value;
body = hid_EmailBody.Value.Replace("@Compting", "HHH").Replace("@toll", hid_TollFreeNo.Value).Replace("@llnk", "<a style='font-family: Tahoma; font-size: 10pt; color: #800000; font-weight: bold' href='http://" + url + "'>click here To Download</a>");