Ruth has stcks and bonds, lets call it S and B
you want to know if S increased over a period of time.

1) tells you that S/(S+B) has increased, this can happen in two ways S increases or S+B decreases, that could occur from B decreasing, so not sufficient.

2) tells you that S+B increased, also could mean only that B increased, so not sufficient

if you put the two together, S+B increased, but so did S/(S+B) you know for sure that S must have increased.

Answer C.