Tallest known gymnosperm is

The correct option is Sequoia.

Gymnosperms are a group of seed-producing plants that do not form flowers or fruits, where the seeds are relatively exposed or "naked". Among all the living gymnosperms, the genus Sequoia holds the record for containing the tallest tree species on Earth.

Here is a step-by-step breakdown of why this option is correct:
1. Sequoia sempervirens (commonly known as the coast redwood) is the tallest known living tree species on Earth. Individual specimens of this gymnosperm can reach heights well over 100 meters.
2. Specifically, a coast redwood named Hyperion, located in Redwood National and State Parks in California, USA, is recognized as the tallest living tree, measuring approximately:

$$115.92\text{meters (}380.3\text{feet)}$$

3. Let us briefly look at the other options to understand why they are not the correct choice for this specific question:
• Pinus: While some pine trees can grow quite tall, they do not reach the record-breaking heights of the coast redwoods.
• Ginkgo: The single surviving species of this group, Ginkgo biloba, is a medium-to-large tree reaching up to 20-35 meters, which is much shorter than Sequoia.
• Ephedra: These are typically small, shrubby gymnosperms, often adapted to arid environments, and are very low in height.

Therefore, Sequoia is scientifically recognized as the tallest known gymnosperm.