All rows generally, but he has three ways of doing it; either by his statements regarding straightening the rows (1), or by him physically straightening the row

Answered by Abu Fajr حفظه الله ___ (1) عَنْ أَنَسٍ رضي الله عنه عَنِ النَّبِيِّ ﷺ قَالَ: ((سَوُّوا صُفُوفَكُمْ فَإِنَّ تَسْوِيَةَ الصُّفُوفِ مِنْ إِقَامَةِ الصَّلاَةِ)).‏ Narrated Anas: The Prophet (ﷺ) said, "Straighten your rows as the straightening of rows is essential for a perfect and correct prayer." [Agreed upon] (2) عن النُّعْمَانَ بْنَ بَشِيرٍ رضي الله عنه يَقُولُ كَانَ رَسُولُ اللهِ ﷺ يُسَوِّي صُفُوفَنَا حَتَّى كَأَنَّمَا يُسَوِّي بِهَا الْقِدَاحَ حَتَّى رَأَى أَنَّا قَدْ عَقَلْنَا عَنْهُ. Nu'man ibn Bashir (May Allah be pleased with them) narrated: Messenger of Allah (ﷺ) used to straighten our rows (in Salat), as if he was straightening an arrow with their help until he saw that we had learnt it from him. [Muslim 436] (3) روى مالك في الموطأ: حدثني مالك عن عمه أبي سهيل بن مالك - عن أبيه أنّه قال: كنت مع عثمان بن عفان - رضي الله عنه - فقامت الصلاة، وأنا أكلمه في أن يفرض لي، فلم أزل أكلمه، وهو يسوّي الحصباء بنعله، حتى جاء رجال قد كان وكلهم بتسوية الصفوف، فأخبروه أنَّ الصفوف قد استوت، فقال لي: استو في الصف، ثم كبر. ...some men came, who he (Uthman) had tasked with straightening the rows. They informed him that the rows were straight, so he said to me, Stand in line, and he made takbir. [Al-Muwatta' - authenticated by Sheikh Albāni]