Isomerism

3 b

convert to fischer projection and then convert that to newman

c

c

c

H bonding ho agar kahi pe bhi then gauche is the answer

a

Due to ring flipping on N, it forms a racemic mixture and thus its optically inactive

B is active as S has a lp

d

i is chiral as there’s an allene so the other part of the db is in a plane perpendicular to the rest of the molecule

so it becomes chiral??

d