Homeostasis is the body's ability to maintain constant conditions when conditions change.
a. internal; external
b. external; internal
c. oxygen; temperature
d. temperature; ph

The  best answer is a - internal : external
Homeostasis is the body's ability to maintain constant internal conditions when external conditions change.

It is the ability of an organism or  a cell to regulate its internal conditions, usually through a system of feedback controls so as to stabilize the health and functioning regardless of the outside changing of conditions..

For example in man, homeostasis occurs when the body regulates body temperature  in an effort to maintain an internal temperature at around 37° C.