Final answer:
The element in period 6, group 8A is not given in the choices provided, as they belong to different groups. Arrange elements oxygen, phosphorus, potassium, and sulfur in order of increasing atomic size using periodic trends; oxygen is smallest, followed by sulfur, then phosphorus, and potassium is the largest.
Step-by-step explanation:
The element located in period 6 and group 8A (or 18, using the IUPAC designation for noble gases) of the periodic table is not among the choices of Oxygen, Sulfur, Selenium, or Tellurium as they all belong to different groups. Group 8A elements are noble gases and none of the options provided are noble gases. However, if we are to consider the elements provided in regards to an inquiry about periodic trends such as atomic size, considering the periodic table layout, we can determine the increasing size among the options given.
Atomic size generally increases from top to bottom within a group and decreases from left to right across a period. Therefore, when arranging the elements oxygen, phosphorus, potassium, and sulfur in order of increasing atomic size, we start with the smallest, which would be oxygen (as it is furthest to the right and towards the top of the periodic table), followed by sulfur (which is below oxygen in the same group), then phosphorus (which is to the left of sulfur and oxygen), and finally potassium (which is in the same period as sulfur and phosphorus but further to the left, indicating it is the largest of the four).