Learn Icelandic: Hvaða keppandi er fljótastur og sterkastur? - Which competitor is fastest and strongest?

In Icelandic, superlative adjectives (meaning "the most" or "-est") have to match the noun they describe in gender, number, and case. Because keppandi (competitor) is a masculine, singular noun in the nominative case, the adjectives take the strong masculine ending -astur.

How do we build these superlative forms from the basic adjectives?

The basic forms for "fast" and "strong" are fljótur and sterkur. To form the superlative, you drop the regular masculine -ur ending to find the core word stems (fljót-, sterk-) and then add the superlative ending -astur.

Keppandi ends in -andi. Does this mean it's a verb form?

It originally comes from a verb, specifically keppa (to compete). Adding -andi to a verb creates a noun for the person doing that action, very much like adding "-er" in English (compete -> competitor). Nouns ending in -andi are always masculine, which is why we must use the masculine superlative forms fljótastur and sterkastur.

Do I need to make hvaða match the gender of keppandi?

No, you get a lucky break here! The question word hvaða (which) never changes its form. It stays exactly the same regardless of the gender, number, or case of the noun following it.