Computation of Number of Years to Double using Rule of 72:

Number of Years = 72/Annual Growth Rate

= 72/17.5

Number of Years = 4.11 or 4 Years 

Or We calculate it using NPER Function in Excel:

=nper(rate,pmt,-pv,fv)

Here,

NPER = Number of Years = ?

Rate = 17.5%

PMT = 0

PV = 1

FV = 2

Substituting the values in formula:

=nper(17.5%,0,-1,2)

NPER or Number of Years = 4.30 or 4 Years 

So, the correct option is 1st "4 Years".