A is greater.
Work Step by Step
If B is 90% of A, then B is less than one whole part (100%) of A. This means A is greater. For example, let A=100. $90\%(A)=B\longrightarrow$ convert percent to decimal by moving decimal 2 places left. $0.9A=B\longrightarrow$ substitute for A $0.9(100)=B\longrightarrow$ multiply $90=B$ $100\gt90$, so A is greater.