Many protist alternate between diploid and haploid stages during their life cycle in a process known as (a) alternation of generations (b) conjunction (c) sexual reproduction (d) asexual reproduction

The correct answer is "alternation of generations".

Alternation of generations is a reproductive cycle demonstrated by some plants, fungi, and protists. In protists, it starts with a sporophyte (diploid; 2n) that undergoes meiosis to produce spores which are haploid (n) reproductive cells. These haploid spores undergo mitosis to produce gametophytes (still haploid). These gametophytes will release haploid gametes wherein two will fuse to produce a diploid zygote. This zygote will undergo mitosis to produce a sporophyte. This is the life cycle of some protists which exemplify alternation of generations.