The correct answer is Firm.
Millie was getting old now, but she still enjoyed chasing a tennis ball occasionally. As she dropped the ball adoringly at Peter's feet, he
ruffled the fur around her ears. "We're both getting a little creaky, aren't we, girl?" he murmured. Ten years ago, when he first spotted the
black-and-white puppy at the shelter, Peter had known immediately that this was the dog for him, and the feeling was mutual. Millie
had proved to be a steadfast companion ever since. Last winter, when Peter fell on the ice and twisted his ankle. Millie stayed right by his side.
Licking Peter's face and standing close to help him keep warm, she cared for him until help arrived.
In the above passage, the word that is closest in meaning to "steadfast" is firm.
Some other synonyms of steadfast are:
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