I am taking a guess, but something like this:

-- Setup sample table a and data.

create table #TestData(

ContractNumber int,

StartDate date,

RentalAmount int,

RentalFrequency varchar(10)

);

insert into #TestData

values (12345,'2013-01-01',100,'Quarterly'),

(23456,'2014-02-01',125,'Monthly'),

(34567,'2014-02-01',135,'Quarterly'),

(23456,'2014-02-01',150,'Annual');

with e1(n) as (select 1 from (values (1),(1),(1),(1),(1),(1),(1),(1),(1),(1))dt(n)),

e2(n) as (select 1 from e1 a cross join e1 b),

e4(n) as (select 1 from e2 a cross join e2 b),

eTally(n) as (select row_number() over (order by (select null)) from e4 a cross join e2 b)

select

td.ContractNumber,

rd.RentalDate,

td.RentalAmount

from

#TestData td

cross apply (select top (datediff(month, td.StartDate, getdate()) / case td.RentalFrequency when 'Monthly' then 1 when 'Quarterly' then 3 when 'Annual' then 12 end + 1)

dateadd(month, (n - 1) * case td.RentalFrequency when 'Monthly' then 1 when 'Quarterly' then 3 when 'Annual' then 12 end, td.StartDate)

from

eTally) rd(RentalDate);

-- drop test data

drop table #TestData;