In service marketing, apart from traditional four elements of marketing mix, there are three more elements, namely, people, process and ‐?‐.

i agreed with @Devabrat Jha 

Devabrat Jha Business Manager - Professional services at Pixel Digital Systems LLC

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The fourth traditional  marketing are Product, Price, Place and Promotion.

The three additional elements are : People, Process and Physical Evidence. 

Then my response would be A/Physical Evidence

(a) is the correct answer (physical evidence).

The4 traditional Ps are product, price, place, and promotion.

A) Physical Evidence

It is the important element in Service Marketing. As services are mostly Intangible thus largely customers tent to believe mostly on tangible cues to evaluate the product  to make the decision of buying it. Therefore, marketer's developed 'Physical Evidence" to replace the 'Physical Cue' in the service.

Physical Evidence is the "Material part of service".

d