Mysterious Organisms Q4

I am having trouble understanding question 4 logic, can someone help?

I am trying to randomly select a base in dna array with baseAtIndex and then assign it as the new base (unless this new randomly selected base is not the same as returnRandBase()). But it still doesn’t make much sense as I’m not utilizing specimenNum that way.

 const pAequorFactory = (num,arr) =>{
   return {specimenNum: num,dna: arr, mutate() {
//randomly select a base
   const baseAtIndex = this.dna[Math.floor(Math.random() * dna.length)];
//assign a new base 
    const newBase =  returnRandBase();
    if (newBase!=baseAtIndex) {return newBase;}
    else {return 'Try again'};