Which era saw the spread of true conifers?

The Jurassic terrestrial environments saw an increase in gymnosperm groups such as the cycads, ginkgoes, and cycadeoids. Reptilian groups also became more adept at flying, while the dinosaurs grew in size with such giants as Apatosaurus and Brachiosaurus, and ferocious predators like Allosaurus. The first birds appeared, as did the most famous fossil bird, Archaeopteryx.

Plants

The Jurassic was a time of increasing diversity for the cycads and a similar-looking group, the cycadeoids. The cycadeoids, sometimes known as the Bennettitales, produced leaves that superficially resembled cycad leaves. However, details of the reproductive structures indicate the two were not closely related. During the Mesozoic leaves of these groups were so common that paleobotanists refer to the era as the age of cycads.

Cycads

The modern cycads are a pale remnant of the dominant plants of the Mesozoic today confined to the tropics. Cycads retain some fern-like features, notably pinnate leaves (feathery looking) and circinate vernation. However, cycads normally produce cones of reproductive structures. These cones are unisexual, in fact the plants are dioecious, having separate male and female plants. Based on structure of the leaf cuticle, cycads are usually separated from the cycadeoids. A number of leaf genera from the Mesozoic have been assigned to the cycads: Taeniopteris and Nilssonia.

Cycadeoids

The cycadeoids superficially resemble cycads in having leaf bases that remain on the soft-wood stems long after the leaves die. Unlike the cycads, however, the cycadeoids had bisexual reproductive structures and structure of their stomates more like what is found in angiosperms and another gymnosperm group, the gnetophytes. Their reproductive structures were at one time interpreted as flowers and the cycadeoids were suggested as ancestors to the flowering plants. Recent analyses suggest the cycadeoids and the flowering plants share a common ancestor, but not a direct one ...

Ginkgophytes

The ginkgoes, like the cycads, originated during the late Paleozoic and diversified during the Mesozoic. Beginning during the Jurassic the number of species increased to a maximum of 11 during the Cretaceous before the decline to the single species, Ginkgo biloba that remains today.

The Mesozoic saw the spread of true conifers, including forms similar to Metasequoia and Sequoia, as well as numerous types of pines. A wide variety of extinct gymnosperms were present, including Claytonia and Pentoxylon.

Mesozoic should be the correct answer.