(c)
$y = \left( c_{1} + c_{2} \right)\cos{\left( x + c_{3} \right) - c_{4}e^{x + c_{5}}}$
$y_{1} = - \left( c_{1} + c_{2} \right)\sin{\left( x + c_{3} \right) - c_{4}}e^{x + c_{5}}$
$y_{2} = - \left( c_{1} + c_{2} \right)\cos{\left( x + c_{3} \right) - c_{4}}e^{x + c_{5}} = - y - 2c_{4}e^{x + c_{5}}$
$y_{3} = - y_{1} - 2c_{4}e^{x + c_{5}}$
$y_{3} = - y_{1} + y_{2} - y$
$\therefore$ Differential equation is
$y_{3} - y_{2} + y_{1} - y = 0$
Which is order 3