Step 1: The task.
We must find which works belong to Theophrastus.
Step 2: Who was Theophrastus.
Theophrastus was an ancient Greek scholar, often called the father of botany, who wrote early books all about plants.
Step 3: Checking each title.
A, Vrikshayurveda, is an Indian text written by Surapala, not Theophrastus. B, Species Plantarum, was written much later by Linnaeus.
Step 4: His actual works.
C, Enquiry into Plants, and D, Causae Plantarum, are both classic books written by Theophrastus. So C and D belong to him.
Step 5: Final check.
Out of the four, only C and D match Theophrastus.
Step 6: Conclusion.
Therefore the correct answer is "C, D". \[ \\boxed{\\text{C, D}} \]