Final answer:
In 'Harry Potter and the Half-Blood Prince,' the words 'For Enemies' are written next to the spell 'Sectumsempra' in the Half-Blood Prince's Potions book, suggesting it is meant to be used against foes. Option A.
Step-by-step explanation:
In the Harry Potter series, the spell 'Sectumsempra' is mentioned in 'Harry Potter and the Half-Blood Prince' as one of the spells found in the used Potions book that once belonged to the 'Half-Blood Prince.'
Next to the spell 'Sectumsempra', written in the margins of the page, are the words, 'For Enemies'. This implies that the spell is intended to be used against someone whom the caster considers an enemy.
The spell itself is quite dangerous, causing deep gashes on the target as though they have been slashed with a sword.
So Option A is the correct answer.