A

Reasoning: Among the standard peripheral IV catheter colour codes, grey represents 16-gauge, green 18-gauge, pink 20-gauge and blue 22-gauge. Flow rate rises steeply as gauge number falls; 16-gauge catheters deliver roughly twice the flow of 18-gauge and several-fold more than 20- or 22-gauge, making the grey 16-G cannula the best choice when rapid fluid resuscitation is required for dehydration or diarrhoeal shock [Mustieles, 2020, PMID 32037555; Tordoff & Sweeney, 1990, PMID 2356937].