When two continental plates of similar densities collide,
A. they remain level since densities in both plates are equal.
B. they both push back down into the mantle
C. one plate rides over the other one and forms a trench
D. they buckle upward and form mountains

D. They buckle upward and form mountains.

Explanation:

When two plates (either oceanic or terrestrial) converge and they are of similar density then they will crash into each other and form mountains. A great example of this is when India crashed into the Asian continent and the Himalayas were formed.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.