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JavaScript

var internal = 'WKF1';
var internal2 = 'WKF10';
var internal3 = 'WKF20';

  var obj = {
      Daily: ['WKF1', 'WKF2', 'WKF3', 'WKF3', 'WKF4', 'WKF5'],
      Weekly: ['WKF6', 'WKF7', 'WKF8'],
      Monthly: ['WKF9', 'WKF10']
  }


      function frequency(wkf, obj) {
          for (var k in obj) {
              if (obj[k].indexOf(wkf) > -1) {
                  return k;
              }
          }
          return 'None found';
      }

var result = frequency(internal, obj);
var result2 = frequency(internal2, obj);
var result3 = frequency(internal3, obj);

console.log(result)
console.log(result2)
console.log(result3)