Right answer:
aquellos
The demonstrative pronouns, such as este, ese and aquel can act as demonstrative adjectives when placed before a noun. So these adjectives demonstrate a quality about the noun they modify. Here that quality is the location in respect to the speaker or the listener. Aquel indicates something that is far away from both the speaker and the listener. So aquellos is the masculine plural form of aquel. Finally:
Aquellos cangrejos
