An iamb is a metrical foot that consists of one unstressed and one stressed syllable. The line that consists of iambs is the first one:<em> I gave the best of all I have to offer to Sam. </em>We can confirm this by highlighting stressed syllables in all of the lines:
I give the best of all I have to offer Sam.
At the end of the month, I get paid.
Stopping smoking helps your health.
Quarterly figures are plummeting steadily.
We can see that the first line is the only one when an unstressed syllable is always followed by a stressed one, which makes its meter iambic.
