Two tectonic plates under water move apart, and magma fills the space between them. this is an example of

Sea floor spreading

Explanation:

In case of a divergent plate motion, two plates move away from another. Due to this motion, there occurs separation of the seafloor, resulting in the addition of new materials to the crust along the mid oceanic ridge. This mid oceanic ridge marks the boundary of separation of the oceanic plates. It often generates deep focus earthquakes and forms volcanoes or chains of volcanic arcs.

Thus, the separation of the two plates under water causes seafloor spreading,  and the magma fills the space along the mid oceanic ridge.

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.