Mutations Are Assumed to Be Independent and Identically Distributed Across Species

The correct answer is:

'a. assumed'

The sentence should be: 'Mutations are assumed to be independent and identically distributed across species.'

In this context, 'assume' is the correct verb form to use. It indicates that the assumption of mutations being independent and identically distributed across species is being made. Therefore, the correct option is 'a. assume'.