To The Hilt

Hello...

There is something difficult for me.

1. In the passage below, disregarding the correctness of the usage of "that", what does "that" refer to?

A. her son's instincts
B. being accustomed to the solitary nature of her son’s instincts

2. Is the "stifle" a verb or a noun?

I smiled. My mother’s reaction to my leaving my wife had been as always calm, unjudgmental and accepting. It was our own business, she had implied. She had also, I thought, understood. Her sole comment to me had been, ‘Solitary people are never alone,’ an unexpected insight that she wouldn’t enlarge or explain, but she had long been accustomed to the solitary nature of her son’s instincts, that I had tried -and failed – to stifle. (To The Hilt; by Dick Francis; 2012)