Concept:SI unit names are written in lowercase, while their symbols may be capitalised.
Solution:The SI unit of energy is the joule.
Its symbol is
J.
In normal writing, the unit name is written as “joule”, not “Joule”.
Key point: Options A and B differ only in the capital letter. By the SI rules a unit name is a common noun written in small letters, even when it honours a person (James Prescott Joule): joule, newton, watt. Only the symbol takes a capital:
J. So 'Joule' (A) and 'j' (C) are both wrong, and Js (joule-second) is the unit of a different quantity.
Answer:Option B (joule)