Answer: I think it's
D. It's important that my sister stop arguing.
(note: I'm conflicted between A and D).
It is best (that)... and It is important (that)... are common usages of the subjunctive mood.
Explanation:
D. It's important that my sister stop arguing.
- Subjunctive is often shown by using "that" to express importance or urgency. This option is correctly using the subjunctive mood. It's implied that she is arguing and she may or may not stop arguing.
B. If I was rich, I would buy everyone pizza.
- This is conditional; it shows cause and effect. IF does not go with WAS; to be subjunctive: IF goes with WERE. This option is incorrect.
C. The doctor asks that bill does something.
- The doctor asks (not asked) bill to take off his shoes. It's not a demand/command, however. This option is incorrect.
A. It is best that she plans... is incorrect usage of the subjunctive mood. It should be plan instead of plans. It is advising, not desiring something.
Apologies in advance since I'm not great at this, but I hope this helps somewhat.
