Which is the topic sentence of the following paragraph?

On the day of my piano recital, I was experiencing nervousness. I felt as if grasshoppers were jumping on my skin. Although I had been practicing the waltz I was going to play, I was afraid the worst would happen. As the day wore on, I grew increasingly fearful. I couldn’t concentrate on anything until the moment of my performance finally arrived.
Question 18 options:

a)

As the day wore on, I grew increasingly fearful.

b)

Although I had been practicing the waltz I was going to play, I was afraid the worst would happen.

c)

On the day of my piano recital, I was experiencing nervousness.

d)

As the day wore on, I grew increasingly fearful. I couldn’t concentrate on anything until the moment of my performance finally arrived.