Which statement describes the trend in first ionization energy for elements on the periodic table?
it generally decreases down a group because valence electrons are farther from the nucleus.
it generally decreases across a period because effective nuclear charge decreases.
it generally increases down a group because valence electrons are farther from the nucleus.
it generally increases across a period because effective nuclear charge decreases.

The statement that best describes the trend in first ionization enery of elements on the periodic table is:

It generally decreases down a group because valence electrons are farther from the nucleus.

The first ionization energy measures how difficult is to release an electron from the outermost shell. The higher the ionization energy the more difficult it is to release an electron, the lower the ionication energy the easier to release an electron.

As the atomic number of the atom increases (which is what happens  when you go down a group) the furthest the outermost shell of electrons will be (the size of the atoms increases) and so those electrons require less energy to be released, which means that the ionization energy decreases.