The correct answer is D. A field guide to butterflies
Step-by-step explanation:
A field guide is a book that contains illustrations or photographs about different natural elements such as plants, birds or insects, along with concise information about them. The main purpose of these books is to help users to identify the animals or elements they observe. These, along with dichotomous keys are the main sources to identify the species of specific organisms.
This implies the best to identify the species or type of butterflies is a field guide to butterflies as this is specially designed for this purpose. Also, others such as a sketchbook or a butterfly net do not help users to identify species and in the case of a dichotomous key of plants, this is useful to identify plants but not butterflies.