The Answer is:
(d) It was not part of Pierre L'Enfant's 1791 plan for the city.
The circular, colonnaded structure, modeled after the Roman Pantheon, is ideally suited to remember Thomas Jefferson, an amateur architect, who introduced this classic style to Colonial America. Although the memorial is more artistically impressive than the Washington Monument, no one protested that it diminished the standing of the Father of the country. Some, however, complained that it overshadowed the Lincoln Memorial, but no one said anything about Adams. And though construction of the memorial did require cutting some old and beautiful elm and cherry trees, and did close the public beach, which some swimmers carped about, the strongest protest was that the memorial deviated from Pierre L'Enfant's 1791 plan for the city.
Pierre L'Enfant was a French artist and engineer who served as captain in the U.S. Army during the Revolutionary War where he met and became fast friends with General George Washington. On January 24, 1791, when President Washington announced the permanent location of the national capital, L'Enfant jumped at the chance to design it, and in March 1791, was hired to prepare the plans. From the start, the National Mall was meant to be the centerpiece, the "Grand Avenue," which was to run from the Capitol to "a point directly south of the President's House." The Mall was to be about a mile long, “bordered by gardens where important structures, monuments and fountains” were to be built. But L'Enfant's formal design for the Mall was soon forgotten. The land was used over the next century for military or agricultural purposes, including slaughtering cattle. It wasn't until 1901 that L'Enfant’s plans began to take effect.
Today it is all that L'Enfant envisioned and more. In addition to the monuments and memorials, are: the U.S. Capitol, the U.S. Botanic Garden, the National Gallery of Art, and ten Smithsonian Museums. It is, by far, the central point of any sightseeing visit to Washington, DC.