Fluorine-18 decays via positron emission to stable oxygen-18

Steps:

• 18F has atomic number 9 (protons) and mass number 18, so 9 neutrons.
• It undergoes beta-plus (positron) decay, reducing protons by 1 while mass number stays 18.
• Product is 18O with atomic number 8 (protons) and mass number 18.
• Neutrons in 18O: 18 - 8 = 10.

Why D is correct:

• Positron emission conserves mass number but decreases atomic number by 1, yielding 18O with 10 neutrons per the neutron count formula (A - Z).

Why the others are wrong:

• A: Matches neutrons in 18F (9), not the decay product.
• B: Equals protons in 18O (8), confusing protons with neutrons.
• C: Equals neutrons in 18F (9), ignoring the decay change.

Final answer: D