Identifying iambic pentameter which line is written in iambic pentameter? i, my dear, was born to-day – for every atom belonging to me, as good belongs to you the earth can have but earth, which is his due even as herself, o friend!

The line which is written in iambic pentameter is the third option:
The earth can have but earth, which is his due

Iambic pentameter means that there are 5 meters, or 10 syllables in each line. The first option has 7 syllables, the second options has 16, and the final one has 7 as well as the first one. The third option, written by Shakespeare, is the only one that has 10 syllables. 

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