Which of the following nouns is in plural?

Concept: Most English nouns form their plural by adding -s/-es, but two exception classes commonly appear in usage tests: (1) a fixed set of nouns ending in -s that are conventionally classed as "plural in form but singular in construction" -- news, and academic/activity nouns ending in -ics such as electronics, physics, mathematics, and athletics, plus singular game names like billiards, all of which take a singular verb; and (2) irregular plurals formed without adding -s, often through an internal vowel change (foot -> feet, goose -> geese, tooth -> teeth, mouse -> mice).

Application: Checking each option against these two classes shows that only one noun here is formed by the irregular vowel-change pattern -- the rest end in -s but remain singular in usage:

• news -- one of the fixed "plural in form, singular in construction" nouns; singular despite the -s ending ("News is...").

• electronics -- also one of these fixed nouns (with news, physics, mathematics, athletics); takes a singular verb ("Electronics is...") even though it can informally denote devices.

• mice -- formed by the vowel change mouse -> mice, the same irregular pattern as foot -> feet and goose -> geese; a genuine plural noun.

• billiards -- the name of a single game, grouped with the same fixed set; singular despite the -s ending ("Billiards is played...").

Cross-check: Substituting a verb confirms this: "News is", "Electronics is" (in its standard/exam sense), and "Billiards is" all pair with a singular verb, while the irregular-plural noun pairs with a plural quantifier and verb ("many mice are..."), confirming it alone is plural.