The plants that first became dominant in the Mesozoic era after the mass extinctions of the Palaeozoic era were the :

Options

A. ferns
B. horsetails
C. flowering plants
D. gymnosperms

Explanation

To solve the question regarding which plants became dominant in the Mesozoic era after the mass extinctions of the Paleozoic era, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Eras**: Understand the geological time scale, specifically the Paleozoic and Mesozoic eras. The Paleozoic era ended with significant mass extinctions, paving the way for new dominant species in the Mesozoic era.

2. **Understand Plant Dominance**: After the mass extinctions at the end of the Paleozoic era, different groups of plants emerged. It is essential to know which groups of plants were prevalent during the Mesozoic era.

3. **Recognize Key Plant Groups**: The primary groups of plants that became dominant during the Mesozoic era include gymnosperms and angiosperms. However, gymnosperms were the first to dominate after the Paleozoic era.

4. **Confirm the Dominance of Gymnosperms**: Gymnosperms, which include conifers and cycads, were the first group of plants to thrive in the Mesozoic era. They adapted well to the changing climate and conditions following the mass extinction.

5. **Identify Specific Families**: During the Mesozoic era, several modern families of gymnosperms originated, such as Pinaceae (pine family) and others.

6. **Conclude the Answer**: Based on the information gathered, the plants that first became dominant in the Mesozoic era after the mass extinctions of the Paleozoic era were gymnosperms.

### Final Answer:
The correct answer is **gymnosperms**.

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