Choose the word or phrase that best completes the sentence.
I’m ______________________ a swimmer as my sister.
a) Better
b) Good as
c) Not as good
d) So good

Hint: In the above sentence, we can see that the speaker is differentiating between his swimming skills with that of his sister. So the phrase or word to be used here has to establish a comparative difference between the two.

Complete answer:
Words like – good, better, and best are known as comparatives. When we are comparing two things or persons we can use ‘better’ as it would be best suited in such situations. If we are comparing more than two things we can use ‘best’ and if we are highlighting our own or someone’s qualities we can use ‘good’, also good is used with nouns, etc.
Now, if we look into the above-given sentence the speaker is highlighting his sister’s skill by saying that he is not good at it.
Therefore, the correct answer would be an option (c), i.e. not as good because this is the only option that describes the sister as a better swimmer than the speaker.
Good will bring out the speaker’s character as a swimmer rather than compare it with his sister and highlight that she is better. Also, it won’t be able to bring out the difference between the two nouns.
So, option (c) is correct and it also makes sense grammatically.

Note:
The speaker is trying to tell that he is bad at swimming but his sister is good. Use a phrase that brings out a similar meaning of him comparing the difference between the two.