Cells with similar structures and functions work together to form
a. tissues.
b. systems.
c. organisms.
d. organs.

Tissues. 
When large groups of tissues form up, they create tissues which the body will use to form organs.
Here is the cell levels in order for future reference:
Cells - Tissues - Organs - Organ Systems - Organisms

The basic unit of a living organism is a cell. Cells with the same function and structure comprises to makes up a tissue. A group of tissues combined with similar structure and forms makes up an organ. A group of organs with relative function makes up a system. Lastly, a group of system with relative functions make up an organism. To simplify:
Cell--> Tissue--> Organ--> System --> Living Organism.