Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow ?

This snippet names nitric oxide (NO) as a distinct reactive chemical species and describes its chemical activity, showing it is a well‑defined biologically relevant molecule.

A student could combine this with basic physiology knowledge (that the body produces many small reactive molecules) to justify checking whether NO is produced endogenously and has signaling roles in humans.

Defines blood pressure and states that constriction of arterioles increases resistance and causes hypertension, linking vessel diameter to blood flow and pressure.

Using the inverse relationship between vessel diameter and resistance, a student could infer that a substance that dilates arterioles would lower resistance and increase flow, motivating investigation of whether NO performs that role.

Explains that hormones and signals can cause contraction of muscles around small arteries, redistributing blood — an example that chemical signals modulate vessel tone.

A student can generalize that endogenous chemicals alter arteriole smooth muscle and therefore seek whether NO is one such endogenous vasomotor mediator.

Describes blood as the transport medium for many substances, implying that dissolved signaling molecules can reach tissues and affect function.

Given blood transports signaling compounds, a student could reasonably hypothesize and then check if NO (or its precursors) is carried or generated near vessels to change blood flow.