Option 1: Both the statements are individually true and Statement II is the correct explanation of Statement I.

This option states that both statements are true, and statement II explains the reason behind statement I. However, this option is not correct in this case because statement II does not explain or provide any reasoning for statement I.

Option 2: Both the statements are individually true but Statement II is not the correct explanation of Statement I.

This option acknowledges that both statements are true, but it states that statement II is not the correct explanation of statement I. This option is correct because although both statements are true, they are not directly related or dependent on each other.

Option 3: Statement I is true but Statement II is false.

This option correctly identifies that statement I is true, but statement II is false. Diamond is indeed very bright, but it does not have a low refractive index.

Option 4: Statement I is false but Statement II is true.

This option suggests that statement I is false, but statement II is true. However, this option is incorrect because statement I is true; diamond is indeed very bright. Statement II, which claims that diamond has a low refractive index, is false.

Therefore, option 3 is the correct answer.