Is know as a force that pulls an object weight to center of the earth

That force would be gravity.  Gravity is what pulls any object down towards Earth due to the large mass of the Earth.

Gravity. More info on gravity from Wikipedia: “Gravity (from Latin gravitas, meaning 'weight'), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward (or gravitate toward) one another. ”

answer: c.

explanation:

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.