The question requires us to explain why NaOH readily dissociates (and is a strong base), while H2O rarely ionizes.
We can consider the type of bonds forming the molecules NaOH and H2O to explain why one dissociates easier than the other.
NaOH is an ionic molecule - it means that there is an electrostatic interaction between the ions Na+ and OH- and, when this compound is in water, it readily dissociates releasing the ions. The readly realease of OH- ions can explain why NaOH is a strong base.
The interaction in NaOH is ionic because sodium (Na) is a metal and it tends to lose its valence electron to achieve stability.
On the other hand, H2O is a covalent molecule, where two atoms of hydrogen (H) are bonded to the same atom of oxygen (O). This type of bond happens because of the electronic configuration of O (6 valence electrons) and H (1 valence electron): each H atom "shares" one electron with the O atom so they can achieve stability, instead of "donating" their electrons such as in an ionic bond.