Regrettably [A] / Regretfully [B] I have to decline your invitation.

I am drawn to the poetic, sensual [A] / sensuous [B] quality of her paintings.

He was besides [A] / beside [B] himself with rage when I told him what I had done.

After brushing against a stationary [A] / stationery [B] truck my car turned turtle.

As the water began to rise over[A]/above[B] the danger mark, the signs of an imminent flood were clear.

'Decline regretfully' is the correct usage. It means being sorry to decline.
Sensuous means taking delight in beauty. Hence is perfect for 2nd sentence.
Beside means next to. Besides means apart from or and another thing. So 'beside' should be used in given context.
Stationary means 'stand still'. Hence is the correct for usage in 4th sentence.
5th statement can be correctly written as, ' As the water began to rise above the danger mark, the signs of an imminent flood were clear.'

Option e) is the correct answer.