Question is whether shares of stock increased?
1) Over the time period, the ratio of number of shares of stock to the total number of shares of stocks and bonds in Ruth's portfolio increased
$$\frac{stock}{stock+bond}$$
so maybe stocks increased or maybe bond decreased , both cases will increase the ratio,
Hence insufficient
2)Over the time period, the total number of shares of stocks and bonds in Ruth's portfolio increased
so stock+bond increased, again, this is possible when stock increased or maybe only bond increased
Hence insufficient,
Combining 1 and 2
if stock+bond is increased, for ratio to increase stock must increase,
hence sufficient
Correct Ans is :C