In a certain code language, 'do or die' is written as 'su ru ko', 'let us do it' is written as 'ko mo yo zu' and 'this or that' is written as 'ga tu ru'. How will 'die' be written in that language?

su

Given; In a certain code language, 'do or die' is written as 'su ru ko', 'let us do it' is written as 'ko mo yo zu' and 'this or that' is written as 'ga tu ru'. 

So,

do  or  die  =  su  ru  ko

let  us  do  it  =  ko  mo  yo  zu

this  or  that  =  ga  tu  ru

Thus,

Code for or is ru and code for do is ko

And,

Code for die is su.