Answer:
B
Step-by-step explanation:
You start from the highest energy level to the lowest. The arrangement is: 1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p. Each box in the configuration can hold up to 2 electrons. This rule tells you to fill all boxes of one particular subshell with 1 electron first, before double occupying them.