(a) Turmeric is a natural indicator found in curry stain.
(b) When you scrub soap on a yellow curry stain, it turns reddish-brown because soap is a basic solution, which changes the colour of turmeric in the curry stain to reddish-brown. When the stain is cleansed with water, the basic soap is eliminated, and the stain turns yellow once more.
(c) Soap is fundamental in nature.
Final answer: Hence, soap on yellow curry gives reddish color because soap is basic.