$\underbrace{2NO + O_2}_{(X)} \rightleftharpoons \underbrace{2NO_2}_{(Y)} \underset{heat}{\overset{cold}{\longleftrightarrow}} \underbrace{N_2O_4}_{(Z)}$
$X, Y$ and $Z$ can be described as:

• A
  $X \to$ Colourless,$Y \to$ Brown,paramagnetic,$Z \to$ Colourless,diamagnetic
• B
  $X \to$ Brown,$Y \to$ Colourless,diamagnetic,$Z \to$ Brown,paramagnetic
• C
  $X \to$ Colourless,$Y \to$ Colourless,paramagnetic,$Z \to$ Brown,diamagnetic
• D
  $X \to$ Brown,$Y \to$ Brown,paramagnetic,$Z \to$ Brown,diamagnetic