In open Angle Glaucoma which investigation is least useful –
Actually all the investigations, provided in the options, can be used in POAG.

"A good baseline evaluation and a record of all parameters - IOP (Tonometry), perimetly, optic nerve head evaluation and gonioscopy - over the years should be available for proper management".  -- Parson's
"Careful examination of disc changes can be made by direct ophthalmoscopy, slit lamp bimicroscopy and indirect ophthalmoscopy".  —Khurana 4th /e 216

As you all know, indirect ophthalmoscopy is usually not used for central retina (optic disc), it is used for peripheral retina, therefore, it is the single best answer here.
Forget about above discussion, you just remember following information :-

Commonly used investigations in POAG → Gonioscopy, Tonometry, direct ophthalmoscopy / slit lamp biomicroscopy, Perimetry.
Indirect opthalmoscopy
Tonometry
Direct opthalmoscopy
Perimetry