When may a full stop (period) follow a unit symbol?
Never, under any circumstances
Only when the symbol happens to fall at the end of a sentence✓الإجابة الصحيحة
After every unit symbol, because symbols are abbreviations
Only after symbols named for people
الشرح
Unit symbols are mathematical entities rather than abbreviations, so they take no abbreviating full stop. In the sentence 'It is 75 cm long.' the full stop belongs to the sentence, not to the symbol.