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You must have reached this stage by solving the equation: 5(y-1)=r(y-1)

Further,
5(y-1)-r(y-1)=0
(5-r)(y-1)=0 (A)
either(5-r)=0 or (y-1)=0

We need to calculate the value of y. If r is 5 as in option 2, then 5-r would be zero. If (5-r) is zero then irrespective of the value of y, the result of multiplication will be zero. I.e if r =5 then y could be 100 or 1 million. Not sufficient.
In option 1, r may be 5 or -5, then equation A would result in following
if r = 5, then y may be infinite numbers
if r = -5 then y must be 1.
Hence not sufficient.

thanks..

realized that after posting the question

5(y-1)=r(y-1)
1) r=+5

for r=5 => infinite y

for r=-5
5y-5=-5y+5
10y=10
y=1

Insufficient

2) r=5
y=infinite values

from 1&2

r=5
hence y=infinite values

hence E is correct.