Question

why do the cells in all living things need energy?
so they can block sunlight
to fuel their chemical reactions
to help them decompose
to communicate with other cells

Explanation:

Cells need energy to carry out various chemical reactions that are essential for life processes such as metabolism, growth, and repair. Energy is not for blocking sunlight (plants use sunlight for photosynthesis but that's a different process), not for decomposition (which is a process for dead matter), and while communication between cells may involve some energy - related processes, the most fundamental and general reason is to fuel chemical reactions.

Answer:

to fuel their chemical reactions