The answer is convex.
A convex lens is a lens that bulges outward at the center. It is thicker in the middle than at the edges. Convex lenses are used to converge light rays, which makes them useful for magnifying objects.
A concave lens is a lens that curves inward at the center. It is thinner in the middle than at the edges. Concave lenses are used to diverge light rays, which makes them useful for correcting nearsightedness.
A plano-convex lens is a lens that is flat on one side and convex on the other. It is used to correct astigmatism.
A plano-concave lens is a lens that is flat on one side and concave on the other. It is used to correct nearsightedness.
In an internal focussing type of telescope, the lens provided is a convex lens. This is because a convex lens is used to converge light rays, which makes it useful for magnifying objects.