Question

insects have:
3 body sections
head
thorax
2 antennae
6 legs

why do arthropods molt?
to just have a cooler exoskeleton
to scare off other arthropods
to grow a new one as they increase in size

Explanation:

Arthropods have exoskeletons (hard outer coverings). These exoskeletons do not grow with the arthropod. When the arthropod increases in size, the old exoskeleton becomes too small, so they molt (shed the old exoskeleton) to grow a new, larger one that can accommodate their increased size. The other options are incorrect: molting is not for having a "cooler" exoskeleton (a non - biological reason) and not for scaring off other arthropods (molting is a growth - related process, not a defense - related behavior for scaring others).

Answer:

to grow a new one as they increase in size