Does you question have the following choices?
A) The density of the pebbles and sand would decrease.
B) The rock would become a plutonic rock composed mostly of quartz.
C) The rock would become more felsic with a higher concentration of magnesium.
D) The pebbles would become distorted and the sand would be recrystallized.
If so, then the answer is letter D.
> The change that would most likely to occur if this rock became buried deep within Earth’s crust and was subjected to intense heat and pressure, but did not melt is: The pebbles would become distorted and the sand would be recrystallized.