The most solid evidence of the effects of herbs on memory suggests that the ground-up leaves of the tree can be moderately effective in delaying symptoms of mild to moderate Alzheimer's disease.

The most solid evidence of the effects of herbs on memory suggests that the ground-up leaves of the A. ginkgo Biloba tree can be moderately effective in delaying symptoms of mild to moderate Alzheimer's disease.

The missing option are:

A. ginkgo biloba

B. portulaca oleracea

C. echinacea

D. St. John's wort

Explanation:

Ginkgo Biloba's leaves were proved to be effective in lessening the effects of Alzheimer's disease if it is given in high doses to patients. Different studies showed that people with Alzheimer that took this medicine improved in the performance of their everyday life activities and their memory. The component of the ginkgo Biloba leaves that helps with Alzheimer's disease is called  EGb 761, it can have side effects such as headaches or stomachaches.

c. is the correct answer