Which of the following is a diploid stage?

Oogonia are diploid.

Primary spermatocytes undergo meiosis-I (reduction division) and form secondary spermatocytes. These secondary spermatocytes exhibit meiosis-II and form spermatids. Spermatids form later sperms. Primary oocytes undergo meiosis-I (reduction division) and form secondary oocytes and 1^st polar bodies. Secondary oocytes undergo meiosis-II to form ova and 2^nd polar bodies. So secondary spermatocytes, spermatids, sperms, secondary oocytes, ova and polar bodies are haploid.