By making the __approximation__ that all the refraction occurs on the centre plane of the lens

The thin lens equation becomes much simpler than a full lens equation

(where the distance between the actual refracting surfaces is considered)

Obviously (as the name implies) the approximation is __best__ for thin lenses

but it can be used with __reasonable__ success for perhaps surprisingly thick lenses.

Just remember that the equation is making an __approximation__ (even for thin lenses)

and that the approximation becomes more approximate the thicker the lens becomes.

Note that the thickness is considered in relation to the lens diameter,

i.e. it is the thickness to diameter ratio that determines how well the "Thin" lens equation approximates.

The multiple choices in your question are quite far apart,

I would have thought that the thin lens approximation would be accurate __enough__ to pick between them.