Non-metallic character increases across a period

Steps:

• Across a period, elements gain protons and electrons in the same shell, increasing effective nuclear charge.
• This makes it harder to lose electrons (decreasing metallic properties) and easier to gain electrons (increasing non-metallic properties).
• From Group I (metals) to Group VII (halogens), the tendency to form negative ions rises.
• Thus, the property with an increasing trend is anion formation ability.

Why A is correct:

• Electronegativity rises across a period per the periodic law, enhancing the ability to attract electrons and form anions (e.g., halogens gain one electron easily).

Why the others are wrong:

• B: Metallic character decreases as elements become more non-metallic.
• C: Electron shells remain constant within a period.
• D: Reactivity with water decreases for non-metals toward Group VII.

Final answer: A