Step 1: Understand the Objective
The task is to identify which of the provided texts does not primarily focus on agriculture, horticulture, or plant biodiversity. This necessitates understanding the core subject of each text.
Step 2: Analyze Each Text
(A) Manusmṛti: This text is a 'Dharmashastra', dealing with social laws and conduct. While it may contain incidental mentions of land and crops, its principal aim is not agricultural or horticultural instruction.
(B) Mānsollāsa: Authored by King Someshvara III, this is an encyclopedia. It includes a significant section, 'Upavanavinoda', which details gardens and horticulture, thus offering relevant information.
(C) Amarkoṣa: This is a Sanskrit thesaurus. It comprehensively lists vocabulary for plants, trees, crops, and land types, thereby contributing to the understanding of plant biodiversity and agriculture from a lexical viewpoint.
(D) Vṛkṣāyurveda: As its name suggests ("the science of life of trees/plants"), this genre of ancient literature is dedicated to botany, agriculture, horticulture, and plant pathology.
Step 3: Determine the Correct Answer
Upon comparison, the Manusmṛti is the least oriented towards direct information on agriculture, horticulture, and plant biodiversity, given its primary focus on law and social order. Therefore, it is the correct choice.