1) Slot test (what kind of word fits before a plural noun?).
The blank precedes the plural count noun “contributions.” In English noun phrases, this position is typically filled by a determiner (e.g., the, some, my, your) that specifies the noun. Therefore, we need a determiner. “Your” is the second-person possessive determiner and fits perfectly: “Send your contributions …”
2) Grammatical form and function of each option.
(A) you are: a finite verb phrase (subject + auxiliary), not a determiner; it cannot directly modify a noun. “Send you are contributions” is ungrammatical.
(B) your: possessive determiner; it modifies a following noun to indicate possession/association (your report, your files). Grammatically correct and semantically intended: the contributions that belong to the addressee.
(C) you’re: contraction of “you are.” Like (A), it is verbal (you are), not determinative. “Send you’re contributions” is ungrammatical because a verb phrase cannot function as a determiner.
(D) yore: a noun meaning “long ago” (as in days of yore); unrelated in meaning and cannot serve as a determiner before “contributions.”
3) Agreement and idiom.
The idiomatic business/academic request is “Send your contributions at the earliest.” Here “your” correctly signals that the sender expects submissions from the recipient(s). Any verbal form (“you are”/“you’re”) would require a complement clause, not a noun: e.g., “You’re delaying … ; send your contributions …” (two separate clauses).
4) Quick diagnostics (try substitutions).
Replace the noun with another plural noun: “Send your documents / your comments.” These remain correct. Try with (A)/(C): “Send you are documents” or “Send you’re documents” — clearly incorrect.
\[
\boxed{\text{The only grammatical and meaningful choice is (B) \;your.}}
\]