Which of the following herbicides is primarily a systemic herbicide?

Glyphosate is primarily recognized as a systemic herbicide, which means it is absorbed by the plant and translocated throughout its tissues. This allows the herbicide to affect the entire plant, including roots and shoots, leading to a more effective and thorough kill of unwanted vegetation.

Systemic herbicides are especially useful for controlling perennial plants and tough weeds, as they disrupt physiological processes throughout the plant. Glyphosate works by inhibiting specific enzymes involved in the synthesis of essential amino acids, ultimately causing the plant to die.

In contrast, the other options listed are not systemic herbicides. Carfentrazone and Flumioxazin are categorized as contact herbicides, which kill vegetative parts of the plant that they come into direct contact with and do not move throughout the plant's systems. Diquat is also a contact herbicide that causes rapid desiccation of plant tissue upon exposure. Therefore, their action is limited to the parts of the plant that are directly treated, rather than being absorbed and used by the plant as a whole.