Answer:
(BH3 follows the octet rule by dimerizing, as Hadi Kurniawan AR pointed out.) For H and He, an "octet" = 2 electrons. Boron does prefer to follow the octet rule, in that it likes to form borate compounds such as NaBH4. It also is happy to form compounds with elements with lone pairs.
1,3-pentadiene has two double bonds which are conjugated, which undergo electrophilic addition reaction on reacting with 
.
The structure of 1,3-pentadiene is shown in the image.
When strong acid such as reacts with 1,3-pentadiene, the electrophilic addition reaction can occur either on double bond at 1,2-position or at 3,4-position. The reaction that occurs is shown in the image.
B because it can stabilize
My answer to this question is C
