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The 6% number is the annual percentage rate (APR), which is just a convention that banks used. It is simply the interest rates received multiplied by the number of periods in a year. So 6% on a semiannual basis means you get paid 3% every 6 months.
The question is asking “If I have a bond that pays 6% on a semiannual basis, what is the equivalent APR I should shop for if I want an annual or quarterly pay bond?“
To convert, you compound it accordingly to calculate annual and quarterly interest payments, then recalculate the APR.
So if you’re receiving 3% every 6 months,
- to make it annual you’d compound it by 2 periods (6 months * 2): 1.03^2 -1 = 6.09%
- to make it quarterly you’d have to compound it by 0.5 periods (6 months * 0.5): 1.03^0.5 -1 = 1.49%, which is 1.49% * 4 = 5.96%
Bonus info: Because of the nature of compounding, frequent interest payments will always result in a lower APR. So without calculating you can already tell that C is wrong, because the quarterly pay bond APR is higher than the semiannual (6%).
Hope that is clear, English is not my first language.