There are 5 elements that are liquid at 38 degrees Celsius.
Four of these are metals, all of which can be used in thermometers.
Cesium (atomic number 55) and
Francium (atomic number 87) are Alkali metals.
Mercury (atomic number 80) is a Transition metal.
Gallium (atomic number 31) is one of the 10 other types of metal.
There is another Alkali metal that is liquid at a mere 1 degree Celsius more: rubidium (atomic number 37). However, it is not the fifth liquid for this question.
Bromine (atomic number 35), a halogen, is the 5th element that is liquid at 38 degrees Celsius.
^ One does not simply put Caesium/Francium in thermometers