Option 1: Physiology is the study of the functions and processes of living organisms, including plants. It focuses on how plants carry out essential activities such as growth, reproduction, and nutrient uptake. This option does not specifically refer to the physical form and external structure of plants.

Option 2: Anatomy is the study of the internal structure and organization of living organisms. While it does involve the study of plant structures, it includes both internal and external aspects. Therefore, it is not specifically focused on the external structure of plants.

Option 3: Phytomorphology is the correct answer. It is the branch of botany that deals with the study of the physical form and external structure of plants. It includes the study of plant organs such as leaves, stems, and roots, as well as their shapes, sizes, and patterns.

Option 4: Cytology is the study of cells, their structure, and function. It does not specifically focus on the physical form and external structure of plants.

Therefore, the correct answer is option 3 - Phytomorphology.