The correct spelling is "focusing". Here’s a breakdown:
- "Focusing" is the American English spelling and is widely accepted in all contexts. In American English, the spelling typically drops the double 's' when forming the gerund or present participle of verbs (e.g., "focus" becomes "focusing").
- "Focussing" is the British English variant, but it is less common and often considered nonstandard. In British English, some verbs that are two syllables and end in a single vowel followed by a consonant may double the consonant (hence "focussing").
In summary, use "focusing" for standard English writing, unless you are specifically adhering to British English conventions.