The function f is defined for all ...

let n= 9

F(n) = 1,2,4,5,7,8

Similarly,

if p is prime number, lets say 5
f(p) = 1,2,3,4

if p = 7
f(p) = 1,2,3,4,5,6

if p=11
f(p) = 1,2,3,4,5,6,7,8,9,10

Clearly from above pattern we can say that p-1 is the answer.

Hence A

I cant think of a shorter approach than what parallel chase has done by picking some primes and then getting to a genric formula.

ANY PRIME...is the helper in the kitchen:

P=2

f(p) = 1

A) p-1

That's it.

ANY PRIME...is the helper in the kitchen:

Similar approach to what paralle chase did. I am sure we will be checking atleast one more prime (alteast I will) to make sure it is p-1 in the real GMAT.

Like I always say different minds think differently(may be u will stop wiht just 1 and be 100% confident about it )

I got to play catch up for the extra 5-10 secs I took to check with one/two more primes

Regards,
CR

yeah .. thanks guys !