Verified Answer :

Curti

The answer is A
Hide the Oxygen for a minute
Mass of elements:
Be:9.012
Mg:24.305
Ca:40.078
Sr:87.62
Out of all these choices, Be is the lightest

Answer: BeO

Explanation:

percent by mass of oxygen\(=\frac{\text {mass of oxygen}}{\text {Total mass of substance}}\times 100\%\)

percent by mass of oxygen in BeO\(=\frac{16g}{25.01g/mol}\times 100=63.97\%\)

percent by mass of oxygen in MgO\(=\frac{16g}{40.30g/mol}\times 100=39.70\%\)

percent by mass of oxygen in CaO\(=\frac{16g}{56.08g/mol}\times 100=28.53\%\)

percent by mass of oxygen in SrO\(=\frac{16g}{103.62g/mol}\times 100=15.44\%\)