As the elements in Period 2 are considered in order from lithium to fluorine, there is an increase in the
A)
atomic radius
B)
electronegativity
C)number of electron shells
D)
number of electrons in the first shell

B: electronegativity

Explanation:

Electronegativity is the measure of how strongly atoms attract bonding electrons to themselves.

A is incorrect because Atomic radius though decreases as you go from lithium to fluorine of the periodic table. Atomic radius increases when you go from either top to bottom of the periodic table, or if you go from right to left. So answer A is inaccurate.

C is incorrect because in period 2, all the elements have two electron subshells. For example, Al has 13 electrons. So it has two electron subshells. This is since the first subshell has only two valence electrons and while the second subshell in Al has 11 valence electrons. Therefore, it is inaccurate to say this answer.

D is incorrect because in the first shell, you can only have a max of two electrons. The first shell neither decreases or increases. Therefore, D is inaccurate too.

B is correct because as you go from the periodic table from left to right, the electronegativity and ionization energy increases. This is since the more valence electrons an element has, the more electronegative is.  Fluorine for example is desperately trying to get one more electron to have a total of 8 electrons. It wants to have a full shell. Therefore, B is the most relevant answer.