Bacterial conjugation is a unidirectional horizontal gene transfer process discovered by Lederberg and Tatum (1946).
The donor strain ($F^+$ or $Hfr$) must possess the Fertility factor (F-plasmid). The F-plasmid carries the $tra$ (transfer) operon genes that encode the sex pilus (F-pilus) required to establish physical contact and form a mating bridge with the recipient ($F^-$) cell, as well as the relaxosome enzymes that initiate rolling circle replication and strand transfer.

In molecular biology:
$1\text{ kb (kilobase)} = 1000\text{ base pairs (bp)}$ for double-stranded DNA.

Assertion (A) is true: In batch cultures of endospore-forming bacteria like Bacillus subtilis, vegetative cells divide exponentially during the log phase. As growth transitions into the stationary phase, vegetative binary fission ceases and the developmental program of endosporulation is triggered.
Reason (R) is true: The master transcriptional regulator Spo0A is activated by phosphorylation via a multi-component phosphorelay that senses nutritional starvation (carbon, nitrogen, and phosphorus depletion) coupled with high cell density signals (Phr peptide quorum sensing).
Because nutrient starvation and cell density during the onset of stationary phase are the precise signals activating sporulation, Reason (R) is the correct explanation of Assertion (A).

Mycorrhizal fungi (particularly Vesicular-Arbuscular Mycorrhizae / AM fungi of Glomeromycota) produce extensive extra-radical hyphal networks that traverse beyond the root phosphorus depletion zone. They solubilize fixed soil phosphates by exuding organic acids and phosphatase enzymes and absorb immobile orthophosphate ions ($H_2PO_4^-$), dramatically enhancing plant phosphorus nutrition.
Options 1, 2, and 3 (Rhizobium, Azolla-Anabaena, and Frankia-actinorrhizal associations) are biological nitrogen-fixing symbioses, not phosphorus-mobilizing.