As the interphase can be subdivided into three sub-phases (also, simply, phases or stages), namely Gap 1 (G1), S and Gap 2 (G2), the question can be misleading. DNA synthesis/replication occurs only in the S (from Synthesis) sub-phase.
The question seems wrong on two grounds:
Interphase is not "the" phase in which (only) DNA replication occurs. The "S" phase would be the subject of the sentence. To say Interphase is the phase in which DNA replication occurs is to forego cell changes during G1 and G2 phases, rendering this definition an improper one.
Cells may go in a quiescent, G0 phase. Some resources may consider G0 as part of the interphase. Depending on many factors, for a given cell, G0 may be transient or permanent. If it is permanent, well, the interphase in this case may never even deal with DNA replication.
Despite these intricacies, given the context, the question to your problem is nonetheless NOT FALSE (but, also, not true in a clear-cut, objective manner, as one would expect in an exam). You have not provided what the bolded part was (subject to change). Interphase is indeed the longest phase (the other large phase being the cell division - Mitosis/Meiosis - or M-phase, which is shorter). Thus, I cannot see how one may change only a part of the sentence to make it truer (it would take at least two amendments). Going with TRUE will most likely give you the points, unless other wrongful interpretations may have been approached by whoever created this test item.
PS
The G1, S, G2 are not "checkpoints", as in the chosen answer. Those would be G1/S, G2/M, or metaphase checkpoint (most important ones).