Hormone responsible for decidual reaction and Arias stella reaction in ectopic pregnancy is :
Arias stella reaction

Arias stella reaction is characterized by adenomatous change of the endometrial glands.
Cells loose their polarity, have hyperchromatic nucleus, vacuolated cytoplasm and occasional mitosis.
The reaction is seen in ectopic pregnancy (in 10 – 15% cases) and indicates blightening of conceptus be it intrauterine or extrauterine. (therfore it is not specific for ectopic pregnancy)
It occurs under the influence of progesterone.

Decidual reaction :

Decidua is the specialised highly modified endometrium of pregnancy.
Decidual reaction/decidualisation is the conversion of secretory endometrium into decidua and is dependant on estrogen and progesterone.
Decidual reaction is completed only with blastocyst implantation.

So, hormone which is common to both Arias stella reaction and decidual reaction is progesterone which is our answer of choice.
Oestrogen
Progesterone
HCG
HPL