Which of the following best describes someone who is 'implacable'?

The term 'implacable' refers to someone who is unable to be calmed down or appeased; they are steadfast in their feelings, often showing little to no inclination to forgive or relent. This quality embodies a sense of being unyielding and unforgiving, which is why the choice that describes someone as 'unyielding and unforgiving' accurately encapsulates the meaning of the word.

The other options suggest characteristics that contradict the essence of being implacable. For example, someone who is 'easily swayed by others' demonstrates flexibility and susceptibility to influence, contrary to the firm stance of an implacable individual. Similarly, 'constantly seeking satisfaction' indicates a desire for change or resolution rather than the steadfastness implied by implacability. Lastly, a 'person with a forgiving nature' clearly opposes the idea of being unforgiving, highlighting a willingness to let go of grudges, which is not characteristic of someone who is implacable.