Explanation:

The gametophyte generation is the haploid phase of a plant's life cycle, producing gametes through mitosis. In some plants, the gametophyte is an independent, free-living generation, while in others, it is dependent on the sporophyte generation for nutrition and support. The question asks us to identify which plant does not have an independent, free-living gametophyte generation.

Polytrichum:
Polytrichum is a genus of mosses, and mosses are known to have an independent, free-living gametophyte generation. So, it is not the correct answer.

Adiantum:
Adiantum is a genus of ferns, and ferns also have an independent, free-living gametophyte generation. Therefore, it is not the correct answer.

Marchantia:
Marchantia is a genus of liverworts, and liverworts have a gametophyte generation that is not independent and free-living. The gametophyte generation of liverworts is dependent on the sporophyte generation for nutrition and support. Therefore, Marchantia could be a possible answer.

Pinus:
Pinus is a genus of gymnosperms, specifically conifers. Conifers have a dependent gametophyte generation that is not independent and free-living. The gametophyte generation of conifers is highly reduced and is contained within the cones of the sporophyte generation. Therefore, Pinus is the correct answer to the question.

In conclusion, among the given options, the plant in which the gametophyte is not an independent, free-living generation is Pinus.

Unlike bryophytes and pteridophytes, in gymnosperms (e.g., Pinus, Cycas, etc.), the male and female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.