You can use either of one:
(1)
Application.StartupPath - that returns bin/Debug folder path.
(2)
Environment.CurrentDirectory - that also returns bin/Debug folder path.
as shown in code snippet:
// new report document object
ReportDocument rpt = new ReportDocument();
string reportPath = Application.StartupPath + "\\" + "Client.rpt";
// loading the ItemReport in report document
rpt.Load(reportPath);