Similar question was previously asked on these topic. Also, Wolfram Mathworld has the same definition as the question that the proper divisors of a number are the divisors excluding the number itself.
But this site has excluded $1$, too, from the number of proper divisors. Same here (in the section "Problems Based on Number Theory"). In my maths textbook "Objective Mathematics Vol.1" by R.D. Sharma this illustration is given which is same as these two.
My question is simple: Who is correct?