Two groups of tomatoes were grown under laboratory conditions, one with humus added to the soil and one a control without humus. The leaves of the plants grown without humus were yellowish (less green) compared with those of the plants grown in humus-enriched soil. The best explanation is that a. the healthy plants used the food in the decomposing leaves of the humus for energy to make chlorophyll. b. the humus made the soil more loosely packed, so water penetrated more easily to the roots. c. the humus contained minerals such as magnesium and iron needed for the synthesis of chlorophyll. d. the heat released by the decomposing leaves of the humus caused more rapid growth and chlorophyll synthesis.

Verified step by step guidance

1. Step 1: Understand the problem. The problem is asking for the best explanation for why the leaves of the plants grown in humus-enriched soil were greener than those grown without humus.
2. Step 2: Review the options. Option a suggests that the plants used the food in the decomposing leaves for energy to make chlorophyll. Option b suggests that the humus made the soil more loosely packed, allowing water to penetrate more easily to the roots. Option c suggests that the humus contained minerals such as magnesium and iron needed for the synthesis of chlorophyll. Option d suggests that the heat released by the decomposing leaves of the humus caused more rapid growth and chlorophyll synthesis.
3. Step 3: Evaluate each option. Option a is unlikely because plants make their own food through photosynthesis, they don't rely on decomposing leaves for energy. Option b is plausible, but it doesn't directly explain why the leaves are greener. Option d is unlikely because heat doesn't directly contribute to chlorophyll synthesis.
4. Step 4: Choose the best option. Option c is the most likely explanation. Chlorophyll, the pigment that gives plants their green color, requires certain minerals for its synthesis, including magnesium and iron. If the humus added to the soil contained these minerals, it would provide the plants with the necessary nutrients for chlorophyll synthesis, resulting in greener leaves.
5. Step 5: Final answer. The best explanation is that the humus contained minerals such as magnesium and iron needed for the synthesis of chlorophyll.