What is the difference between a missense mutation and a silent mutation..

Missense mutation comes under point mutation. In which single a single nucleotide gets changed which codes for the different aminoacids or simply  the change in one pair of DNA. It is called non - synonymous mutations.

In silent mutation there is a change occur in DNA codon which never effect the amino acid translation and simply there is no effect or change in phenotype of a particular individual.

we would need to see the picture to answer it

c. earths continental crust is thicker than its lithosphere.

because, the lithosphere is the outermost layer of the earth/ 100 km thick and is defined by its machanical properties.the lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. the lithosphere is the coolest layer of the earth in terms of temperature. the term lithosphere should not be confused with the use of "geosphere" which is used to indicate all of earth systems, including the atmosphere, hydrosphere   and biosphere. the surface of the earth can be divided into the continental crust and the oceanic crust.   the continental crust is made mostly of granite while the oceanic crust is made of basalt. the average thickness of the continental crust is 25 miles and the average thickness of the oceanic crust is 5 miles.   three types of rock, igneous, sedimentary, and metamorphic, make up the surface of the earth. that's why 3 option is preferable.