A reaction that is spontaneous happen,

A. could
B. will never
C. will partially
D. will always

A reaction that is spontaneous will always happen. Option D is right.

Explanation:

When certain conditions for reactions are fulfilled the reaction that occurs without any outside intervention with the formation of product is called as spontaneous reaction.

In spontaneous reaction the product is of lesser energy than reactants.

The spontaneous reaction leads to increase in entropy. If the entropy of the system and surrounding is positive the reaction is spontaneous.

During phase change as solid to liquid causes molecules to be disordered and increase in the movement causes increase disorder hence increase in entropy and a spontaneous reaction.

In chemical reactions if on mixing the substances heat is released  thus entropy increases. The spontaneous reaction can also occur even if entropy decreases.

The reaction is not spontaneous if Gibb's free energy is negative.

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.