Cold Wooden flooring

Just looking for advice.
My property has solid wooden flooring in the ground floor rooms. The tennant is complaining that it is too cold due to the gaps in the wooden flooring.
Am I obliged to change the flooring?
Or is there another solution for this?

line it with a good hardboard, underlay and carpet//or seal the gaps with a coloured filler or silicone //or Take the boards up and insulate underneath