The no. of Ring or cyclic Isomers for (pent - 1 - yne), excluding stereoisom is
Looking at the degree of unsaturation. ( = 2), there can be 2π bonds OR one ring +1π bond or 2 rings.
IF then are two π bonds. The compound will be acyclic.
Hence we look at combinations of 1π bond +1 ring OR 2 ring 1