Half-life determination requires decay data

Steps:

• Identify the initial activity or amount of the isotope from given data (e.g., graph or table).
• Locate the time when the activity or amount halves the initial value.
• That time interval is the half-life by definition.
• Compare to options to select the match.

Not enough information: No decay data, graph, or numerical values provided to calculate or identify the half-life.

Why A is correct: Not applicable without data.

Why the others are wrong:

• B: No basis to select 90 minutes over others.
• C: No basis to select 180 minutes over others.
• D: No basis to select 240 minutes over others.

Final answer: Not enough information.