Who built the Red Fort?

Concept: Mughal rulers commissioned distinctive monuments during their reigns; a "who built X" question is answered by matching the specific structure to the ruler whose reign and capital-building programme produced it, not just any Mughal-era ruler.

Application: The Red Fort (Lal Qila) in Delhi was commissioned by Shah Jahan starting in 1638 as the fortified palace complex at the heart of his newly founded capital, Shahjahanabad (present-day Old Delhi), and was completed around 1648. It went on to serve as the principal residence of the Mughal emperors for roughly two centuries afterward.

Cross-check: Sher Shah Suri (an Afghan ruler, not a Mughal, reigned 1540-1545) is remembered for administrative reforms and for renovating and extending the ancient trade route that became the Grand Trunk Road (Sher Shah Suri Marg), not this complex. Humayun (reigned 1530-1540, 1555-1556) is chiefly associated with his own tomb in Delhi, commissioned by his senior widow, Empress Bega Begum, after his death -- an earlier-generation memorial, not a new-capital fortress. Akbar (reigned 1556-1605) built Agra Fort and the city of Fatehpur Sikri, both far from this particular Delhi complex. None of these three rulers' major building projects match the Red Fort.

Result: Shah Jahan built the Red Fort.