A more conservative and scientifically sound estimate about the total number of species present on earth, was made by
(a) Robert May
(b) Paul Ehrlich
(c) David Tilman
(d) Both A and B

Options

A. Robert May
B. Paul Ehrlich
C. David Tilman
D. Both A and B

Explanation

To solve the question regarding who made a more conservative and scientifically sound estimate about the total number of species present on Earth, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the individual who provided a conservative and scientifically sound estimate of the total number of species on Earth.

2. **Review the Options**: The options provided are:
- (a) Robert May
- (b) Paul Ehrlich
- (c) David Tilman
- (d) Both A and B

3. **Identify Key Information**: The video transcript mentions that Robert May provided a more scientific and conservative estimate of global species diversity, placing it at around 7 million species.

4. **Evaluate Each Option**:
- **Option (a) Robert May**: He is credited with providing the conservative estimate of around 7 million species.
- **Option (b) Paul Ehrlich**: While he is a notable figure in biodiversity, the transcript does not mention him providing a specific estimate.
- **Option (c) David Tilman**: There is no mention of him providing an estimate in the transcript.
- **Option (d) Both A and B**: Since only Robert May is mentioned as providing the estimate, this option is incorrect.

5. **Conclusion**: Based on the information provided, the correct answer is **(a) Robert May**.

### Final Answer:
(a) Robert May