Ageologist examines the chart and claims to know which magma sample is andesitic. which sample is the geologist referring to?

Hopefully the chart you referred to is the one attached in the image below.

Then the correct answer will be magma sample B. Containing more than 60% of silica and a high water vapour level.

Andesite is a class of fine-grained intermediate igneous rock, of volcanic origin, containing mostly plagioclase feldspar. They contain between 57% to 63% of Silicon oxide (SiO2). They are also a major component of the Martian crust