cannot be used as a function

I am trying to create a function that converts the temperature read in by a dht sensor. But I keep receiving an error ‘kelvin’ cannot be used as a function. I followed this tutorial How to Set Up the DHT11 Humidity Sensor on an Arduino

#include <dht.h>
#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

dht DHT;

#define DHT11_PIN 7

void setup() {
  lcd.begin(16, 2);
}

void loop()
{
  int chk = DHT.read11(DHT11_PIN);
  float t = DHT.temperature;
  lcd.setCursor(0, 0);
  lcd.print("Temp: ");
  lcd.print(kelvin(t));
  lcd.print((char)223);
  lcd.print("K");
  lcd.setCursor(0, 1);
  lcd.print("Humidity: ");
  lcd.print(DHT.humidity);
  lcd.print("%");
  delay(2000);
}
float fahrenheit(DHT.temperature) {
  int f = (DHT.temperature * 9/5) + 32;
  return f;
}
float kelvin(DHT.temperature) {
  int k = DHT.temperature + 273.15;
  return k;
}

Please include the entire error message. It is easy to do. There is a button (lower right of the IDE window) called "copy error message". Copy the error and paste into a post in code tags. Paraphrasing the error message leaves out important information.

float kelvin(DHT.temperature) {
There is passing by ‘value’ an by ‘reference’.
Is this a proper way to pass a variable to a function ?

Was that the first or last error in the listing? Look at the first one. It may have to do with declaring a function that returns a float

float fahrenheit(DHT.temperature) {

but an attempt is made to return an int

  int f = (DHT.temperature * 9/5) + 32;
  return f;

The fact you are assigning the conversion to an int and have the function declared to return a float notwithstanding...

You have not declared the function parameters correctly:

float fahrenheit(DHT.temperature) {
  int f = (DHT.temperature * 9/5) + 32;
  return f;
}
float kelvin(DHT.temperature) {
  int k = DHT.temperature + 273.15;
  return k;
}

Since you are using the global name you have one of 2 choices.

This:

float fahrenheit() {
  int f = (DHT.temperature * 9/5) + 32;
  return f;
}
float kelvin() {
  int k = DHT.temperature + 273.15;
  return k;
}

or this:

float fahrenheit(float temperature) {
  int f = (temperature * 9/5) + 32;
  return f;
}
float kelvin(float temperature) {
  int k = temperature + 273.15;
  return k;
}