A is correct.
Sustainable growth rate = Retention ratio × ROE.
The higher a company's ROE and its ability to finance itself from internally generated funds (a higher retention ratio), the greater its sustainable growth rate.
In the five-factor ROE, any factor that increases ROE will increase sustainable growth:
ROE = Tax burden × Interest burden × EBIT margin × Asset turnover × Leverage.
A higher tax burden ratio (Net income/Earnings before tax) implies that the company can keep a higher percentage of pretax profits; this implies a lower tax rate and a higher ROE. The interest burden ratio is earnings before tax to EBIT, and a lower ratio means that the company has higher borrowing costs (it gets to keep a lower pre-tax income from a given EBIT), implying a lower ROE and sustainable growth.