Nomenclature of tetrahydropyran ring
There are two isomers of pyran, 2​H-pyran and 4​H-pyran.

    

In principle, it is possible to describe the corresponding saturated compounds using the prefix ‘hydro’, which yields the systematic names ‘3,4,5,6-tetrahydro-2​H-pyran’ and ‘2,3,5,6-tetrahydro-4​H-pyran’. However, these two names actually describe the same compound. In this case, the correct choice is ‘3,4,5,6-tetrahydro-2​H-pyran’ because ‘2​H’ is lower than ‘4​H’ (note that indicated hydrogen atoms have priority over ‘hydro’ prefixes for low locants).

All locants are omitted in compounds or substituent groups in which all substitutable positions are completely substituted or modified, for example, by hydro, in the same way. Thus, in this case, the systematic name is simplified to ‘tetrahydro-2​H-pyran’. Furthermore, omission of indicated hydrogen is permitted in general nomenclature if no ambiguity would result. Thus, in this case, a correct name is just ‘tetrahydropyran’.

Anyway, according to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), preferred IUPAC names of saturated heteromonocyclic compounds are formed by the extended Hantzsch–Widman system. Therefore, the preferred IUPAC name of the given compound is ‘oxane’.