In this question, we will take time to be "x".
And, number of active customers to be "y".
If the slope of a function is constant, then it is a linear function, else it is a non-linear function.
Let consider
Channel A:
Consider the two points (2,250) and (4,410)
Slope=Change in yChange in x=410 − 2504 − 2=80
Now, consider the points (4,410) and (6,570)
Slope=Change in yChange in x=570 − 4106 − 4=80
Since the slope between any two consecutive points are same, the function represented by Channel A is linear.
Slope of function representing Channel A = 80
Let consider
Channel B:
Consider the points (2,250) and (4,250).
Slope=Change in yChange in x=250 − 2504 − 2=0
Since the slope between any two consecutive points is zero, the function represented by channel B is linear.
Slope of function representing Channel B = 0
Let consider
Channel C:
Consider the points (2,250) and (4,360)
Slope=Change in yChange in x=360 − 2504 − 2=55
Now, consider the points (6,490) and (4,360)
Slope=Change in yChange in x=490 − 3606 − 4=65
Since the slope between any two consecutive points are not constant throughout, the function represented by Channel C is not linear.