First Principles Thinking: core idea
A is correct. In the CFA Curriculum, the floater's periodic cash payment is the annual reference rate plus quoted margin, applied to face value and divided by the number of payment periods per year. That gives $PMT = \\dfrac{(MRR + QM) \\times FV}{m}$. The division by $m$ converts the annual rate into a periodic payment. Applying that rule gives the correct floater coupon amount each period.
Why top distractor is wrong (PDF-based misconception): B is tempting because $DM$ also appears in the floater model, but it belongs in the discount rate used to value cash flows, not in the coupon payment itself.
Why remaining distractor is wrong: C multiplies by $m$ instead of dividing by $m$, so it overstates the periodic payment rather than converting the annual rate to one period.