All the carbon atoms in cyclopropane are sp3 hybridized, are coplanar, and all have C-C bond angles of 60o. Nevertheless, the conformation of cyclopropane is very unstable, due to a. steric strain
b. delocalization of electrons
c. angular strain
d. torsional strain

i do not get it   at all

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.