i uploaded the further information. so i hope to check this question again.i don't know the reason that 'b' is a correct answer. because in the book there are the example only about price-weighted index, and s&p is a value-weighted index. so i don't know the s&p(value-weighted)'s stock split mechanism.

Stock Indexes In calculating the Standard & Poor's stock price indexes, how are adjustments for stock splits made? a. By adjusting the divisor. b. Automatically, due to the manner in which the index is calculated. c. By adjusting the numerator. d. Quarterly, on the last trading day of each quarter. Stock Indexes In calculating the Standard & Poor's stock price indexes, how are adjustments for stock splits made? a. By adjusting the divisor. b. Automatically, due to the manner in which the index is calculated. c. By adjusting the numerator. d. Quarterly, on the last trading day of each quarter