Answer:
(a) The opportunity cost of Chinese food has decreased because Indian restaurant raises its price, so Chinese food is more affordable and indian food become expensive.
(b) The opportunity cost of eating a bagel has increased because the price of muffins falls.
(c) The opportunity cost of studying has increased because his wages increases from $10 to $15, so now he have to foregone more money income than before.
(d) The opportunity cost of taking the bus has increased because bus travel becomes more affordable or cheaper due to lower gasoline prices.