The correct answer is option 1: Cell.
A cell is the basic structural and functional unit of living organisms. It is the smallest unit of life and is capable of carrying out all the necessary functions for survival. Cells can exist as single-celled organisms, such as bacteria, or they can come together to form multicellular organisms, such as plants and animals.
Option 2: Tissue is incorrect because tissues are made up of a group of cells with a similar structure and function. While tissues are important in the overall structure and function of an organism, they are not the basic unit.
Option 3: Organ is incorrect because an organ is made up of different types of tissues working together to perform a specific function. Organs are composed of cells, but they are not the fundamental unit.
Option 4: Systems is incorrect because systems are composed of multiple organs working together to carry out a specific function. Systems are made up of organs, which are made up of tissues, which are made up of cells. Therefore, systems are not the basic unit.
In conclusion, the correct answer is option 1: Cell, as it is the basic structural and functional unit of all living organisms.