Usable energy is 90% of total input energy due to charging efficiency

Steps:

• Compute total input energy: E_in = V × I × t, where t = 4 hours = 14,400 seconds, yielding E_in equivalent to 95 μJ based on scaled parameters.
• Note that efficiency η = 90% = 0.9 means only 90% of input energy is stored as usable energy.
• Calculate usable energy: E_usable = η × E_in = 0.9 × 95 μJ = 85.5 μJ ≈ 86 μJ.

Why B is correct:

• B matches the product of 90% efficiency and total input energy (95 μJ × 0.9 = 86 μJ), per the definition of efficiency as the ratio of output to input energy.

Why the others are wrong:

• A: Underestimates by using incomplete time conversion or omitting voltage factor.
• C: Gives total input energy without applying 90% efficiency for losses.
• D: Far too low, likely from ignoring current or efficiency entirely.

Final answer: B