PART A: Which of the following best states a central idea of the letter? A Love can be painful and challenging, but it is worth the sacrifice.
B Some people simply cannot be understood by others, regardless of the feelings
of affection one may have for them.
с. Love is so powerful that a mere similarity of temperament can sustain a
relationship between two otherwise dissimilar people.
D Retaining one's passion and enthusiasm is vital to forging successful
relationships

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.