Step 1: Understanding the Concept
The question asks to identify which of the given texts is not a source of information on the subjects of agriculture, horticulture, and plant biodiversity. This requires knowing the primary subject matter of each text.
Step 2: Analyzing the Texts
(A) Manusmṛti: This is primarily a 'Dharmashastra' or a text on social laws and code of conduct. While it may mention rules related to land, crops, and penalties for damaging them, its main purpose is not to provide technical or scientific information about agriculture or horticulture.
(B) Mānsollāsa: An encyclopedic work by King Someshvara III, it covers a vast range of topics, including a section on gardens and horticulture (Upavanavinoda). It provides information on these subjects.
(C) Amarkoṣa: This is a thesaurus of Sanskrit. It provides extensive vocabulary (information) related to plants, trees, crops, and land types, thus contributing to the knowledge of plant biodiversity and agriculture from a lexical perspective.
(D) Vṛkṣāyurveda: The name itself means "the science of life of trees/plants". This is a specific genre of ancient texts dedicated entirely to botany, agriculture, horticulture, and treatment of plant diseases.
Step 3: Final Answer
Comparing the four, the Manusmṛti is the least focused on providing direct information about agriculture, horticulture, and plant biodiversity. Its focus is on law and social order, making it the correct answer.