Imo answer's D.
The fall of the Berlin Wall in September 1989 inspired a period of optimism in Europe and was the precursor to
forging new personal relationships and forging business partnerships between East to West Germany, a healing of past traumas, and a historical reconciliation with countries emerging from former communist regimes.
A.
forging new personal relationships and
forging business partnerships
between East to West Germany - Wrong Idiom - Between X to Y. "Forging" not parallel with rest of non-underlined list..
B.
forging new personal relationships and
forging business partnerships between East and West Germany - "Forging" not parallel with rest of non-underlined list. Also meaning issue. Precursor to new personal relationships in general, and business partnerships between East and West Germany.
C. a forging of new personal relationships as well as business partnerships between
East Germany and West - East Germany and (the) West? Ambiguous. D is clearer, but this is a close second.
D. a forging of new personal relationships and business partnerships between East and West Germany -
Correct! Succinct, parallel and clear.
E. a forging of new personal relationships along with business partnerships
between East to West Germany - Wrong Idiom - Between X to Y.