In a one-trait test cross, the phenotype that disappears in the f1 generation is the a. dominant trait b. negative trait. c. recessive trait d. heterozygote

The answer would be c. recessive trait.

If you imagine a cross involving height: TT for tall, tt for drawf:

The P generation (test cross) means you're crossing a homozygous recessive with a dominant. A TTxtt cross would produce four Tt offspring, all tall (the dominant trait).