Answer
d. iron.
Work Step by Step
The other three options, oxygen, phosphorus, and carbon are all macronutrients while iron is the only micronutrient and plants only require small amounts of it.
Campbell Biology: Concepts & Connections (9th Edition)
by Taylor, Martha R.; Simon, Eric J.; Dickey, Jean L.; Hogan, Kelly A.; Reece, Jane B.
Published by
Pearson
ISBN 10:
0-13429-601-X
ISBN 13:
978-0-13429-601-2
Chapter 32 - Review - Page 663: 2
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