In exponential growth or decay the increase or decrease is not constant , but it increases or decreases exponentially , i.e. in percentages.
Now let's see the options
a) a palm tree growing 8 inches every year
in this the palm tree is growing at the same constant rate , every year it increases 8 inches only , so this is linear growth.
b) putting $100 in a savings account each week
In this also same as part a , the account is getting $100 , every week , again the growth is linear.
c)a retirement savings account growing by 8% each year
Now in this , the growth is by percentage i.e. let the amount initially was $100 , next year it increases by 8% , and will become 108 , then next year 8% will be calculated on 108 not on 100 , So this growth is exponential
d)a car insurance premium decreasing by $50 each year
here also it is decreasing by $50 each year , decay is constant , not exponential
So answer is OPTION -C
