option A just says that the ratio of shares to total increased ie S/(S+B) increased we cannot determine anything about the individual terms from this information. INSUFF

option B says the total number of shares and bonds increased ie s+b increased which could mean that either shares increased, bonds increased or both increased. no further info is given hence INSUFF.

combining both we know that s/(s+b) increased and the denominator increased hence for the whole fraction to increase S will have to increase. SUFF

Hope it helps